Best hard disk drive (HDD)
A hard disk drive (HDD), commonly referred to as a hard drive or hard disk,
is a non-volatile storage device which stores digitally encoded data on rapidly
rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a
device distinct from its medium, such as a tape drive and its tape, or a floppy
disk drive and its floppy disk. Early HDDs had removable media; however, an HDD
today is typically a sealed unit with fixed media.
HDDs were originally developed for use with computers. In the 21st century,
applications for HDDs have expanded beyond computers to include digital video
recorders, digital audio players, personal digital assistants, digital cameras,
and video game consoles. In 2005 the first mobile phones to include HDDs were
introduced by Samsung and Nokia[citation needed]. The need for large-scale,
reliable storage, independent of a particular device, led to the introduction of
configurations such as RAID arrays, network attached storage (NAS) systems and
storage area network (SAN) systems that provide efficient and reliable access to
large volumes of data.
Technology
A hard disk drive stores information on one or more rigid, flat, disks. The
disks are mounted on a spindle, with spaces in between, and a motor on the
bottom end of the spindle. To read and write to the surface of the disks, the
drive uses a small electro-magnet assembly, referred to as a head, located on
the end of an actuator arm. There is one head for each platter surface on the
spindle. The disks are spun at a very high speed to allow the head to move
quickly over the surface of the disk. Towards the other end of the actuator arm
is a pivot point, and at the end is a voice coil, which moves the head. Above
and below each voice coil is a rare earth magnet. This allows the head to move
towards the center of the disk or towards the outside, in a radial pattern.
A cross section of the magnetic surface in action. In this case the binary data
encoded using frequency modulation.The disk controller uses a digital-to-analog
converter to control the flow of electricity through the voice coil(s) located
on the of the actuator arm. The voice coil acts as an electromagnet; it produces
a magnetic field that interacts with magnetic fields of the magnet located above
and below the voice coil, which causes the voice coil to move the actuator arm,
and in turn the head located on the opposite end of the actuator arm. So as the
voice coil is pushed towards one end, the assembly moves the head towards the
center, and when the voice coil is pushed towards the other end, the heads move
towards the outside edge of the disk, or the heads are parked. The
digital-to-analog converter allows the disk controller to move the head in tiny
steps in either direction.
The disks are made of a non-magnetic material, usually aluminum or glass, and
are coated with a very thin layer of magnetic material. Older disks used
iron(III) oxide as the magnetic material, but current disks use a cobalt-based
alloy.
The inside of a hard disk drive with the disk(s) and spindle motor hub removed.
To the left of center is the actuator arm. A read-write head is at the end of
the arm. The head slider is located just behind the head, on the underside of
the arm. The orange wires on the side of the arm, connect the heads to the
drives controller. The pivot point is the round bolt seen just before the metal
plate. The semi-circular metal plate, top left corner, is one of the permanent
magnets that are used in moving the arm. The voice coil is located underneath,
with a second magnet below.The air filter is within the plastic housing in the
bottom left.The magnetic surface of each platter is divided into many small sub-micrometre-sized
magnetic regions, each of which is used to encode a single binary unit of
information. In today's HDDs each of these magnetic regions is composed of a few
hundred magnetic grains. Each magnetic region forms a magnetic dipole which
generates a highly localized magnetic field nearby. The write head magnetizes a
magnetic region by generating a strong local magnetic field nearby. Early HDDs
used the same inductor that was used to read the data as an electromagnet to
create this field. Later versions of inductive heads included metal in Gap (MIG)
heads and thin film heads. In today's heads, the read and write elements are
separate but in close proximity on the head portion of an actuator arm. The read
element is typically magneto-resistive while the write element is typically
thin-film inductive .
Hard disk drives are sealed to prevent dust and other sources of contamination
from interfering with the operation of the hard disks heads. The hard drives are
not air tight, but rather utilize an extremely fine air filter, to allow for air
inside the hard drive enclosure. The spinning of the disks causes the air to
circulate forcing any particulates to become trapped on the filter. The spinning
of the disks, also allows the hard disk heads to float above the surface of the
disk surface using the same air currents. See Bernoulli's principle.
Using rigid disks and sealing the unit allows much tighter tolerances than in a
floppy disk drive. Consequently, hard disk drives can store much more data than
floppy disk drives and access and transmit it faster. In 2007, a typical
enterprise, i.e. workstation HDD might store between 160 GB and 1 TB of data (as
of local US market by July 2007), rotate at 7,200 or 10,000 revolutions per
minute (RPM), and have a sequential media transfer rate of over 80 MB/s[citation
needed]. The fastest enterprise HDDs spin at 15,000 RPM, and can achieve
sequential media transfer speeds up to and beyond 110 MB/s. Mobile, i.e., Laptop
HDDs, which are physically smaller than their desktop and enterprise
counterparts, tend to be slower and have less capacity. In the 1990s, most spun
at 4,200 RPM . In 2007, a typical mobile HDD spins at 5,400 RPM, with 7,200 RPM
models available for a slight price premium.
Hard drives are precise devices, moving at very high speed, and a number of
analogies have been made to try to describe this. One analogy, proposed by Scott
Mueller, says:
“ ...to such a scale, the heads in this typical hard disk would be about [...]
equal to the Sears Tower if it were tipped over sideways. These skyscaper sized
heads would float on a cushion of air that to scale would only be 5mm thick
(about 0.2") while [...] circling the Earth once every 5 seconds! ”
—Scott Mueller, Upgrading and Repairing PCs, 17th edition
Capacity and access speed
PC hard disk drive capacity (in GB). The plot is logarithmic, so the fit line
corresponds to exponential growth.The exponential increases in disk space and
data access speeds of HDDs have enabled the commercial viability of consumer
products that require large storage capacities, such as the TiVo personal video
recorder and digital music players. In addition, the availability of vast
amounts of cheap storage has made viable a variety of web-based systems with
extraordinary capacity requirements, such as the search and email systems
offered by companies like Google.
The main way to decrease access time is to increase rotational speed, while the
main way to increase throughput and storage capacity is to increase areal
density. A vice president of Seagate Technology projects a future growth in disk
density of 40% per year. Access times have not kept up with throughput
increases, which themselves have not kept up with growth in storage capacity.
As of 2006, disk drives include perpendicular recording technology, in the
attempt to enhance recording density and throughput.
The first 3.5" HDD marketed as able to store 1 TB is the Hitachi Deskstar
7K1000. The drive contains five platters at approximately 200 GB each, providing
935.5 GiB of usable space. Hitachi has since been joined by Samsung and Seagate
in the 1 TB drive market.
Standard Name Width Largest capacity to date (2007) Platters (Max)
5.25" FH 146 mm 47 GB 14
5.25" HH 146 mm 19.3 GB 4
3.5" 102 mm 1.2 TB 5
2.5" 69.9 mm 320 GB 3
1.8" (PCMCIA) 54 mm 160 GB
1.8" (ATA-7 LIF) 53.8 mm
Capacity measurements
The capacity of an HDD can be calculated by multiplying the number of cylinders
by the number of heads by the number of sectors by the number of bytes/sector
(most commonly 512). On ATA drives bigger than 8 gigabytes, the values are set
to 16383 cylinder, 16 heads, 63 sectors for compatibility with older operating
systems. It should be noted that the values for cylinder, head & sector reported
by a modern drive are not the actual physical parameters since, amongst other
things, with zone bit recording the number of sectors varies by zone.
Hard disk drive manufacturers specify disk capacity using the SI prefixes mega,
giga, and tera and their abbreviations M, G and T, respectively. Byte is
typically abbreviated B.
Operating systems frequently report capacity using the same abbreviations but
with a binary interpretation. For instance, the prefix mega can also mean 220
(1,048,576), which is approximately 1,000,000. Similar usage has been applied to
prefixes of greater magnitude. This results in a discrepancy between the disk
manufacturer's stated capacity and what the system reports. The difference
becomes much more noticeable in the multi-gigabyte range. For example, Microsoft
Windows reports disk capacity both in decimal to 12 or more significant digits
and with binary prefixes to 3 significant digits. Thus a disk specified by a
disk manufacturer as a 30 GB disk might have its capacity reported by Windows
2000 both as "30,065,098,568 bytes" and "28.0 GB" The disk manufacturer used the
SI definition of "giga", 109 to arrive at 30 GB; however, because the utilities
provided by Windows define a gigabyte as 1,073,741,824 bytes (230 bytes), the
operating system reports capacity of the disk drive as 28.0 GB.
History
History of hard disk drives
IBM 62PC "Piccolo" HDD, circa 1979 - an early 8" diskFor many years, HDDs were
large, cumbersome devices, more suited to use in the protected environment of a
data center or large office than in a harsh industrial environment (due to their
delicacy), or small office or home (due to their size and power consumption).
Before the early 1980s, most HDDs had 8-inch (20 cm) or 14-inch (35 cm)
platters, required an equipment rack or a large amount of floor space
(especially the large removable-media disks, which were often referred to as
"washing machines"), and in many cases needed high-current or even three-phase
power hookups due to the large motors they used. Because of this, HDDs were not
commonly used with microcomputers until after 1980, when Seagate Technology
introduced the ST-506, the first 5.25-inch HDD, with a capacity of 5 megabytes.
In fact, in its factory configuration, the original IBM PC (IBM 5150) was not
equipped with a hard disk drive[citation needed].
Most microcomputer HDDs in the early 1980s were not sold under their
manufacturer's names, but by OEMs as part of larger peripherals (such as the
Corvus Disk System and the Apple ProFile). The IBM PC/XT had an internal HDD,
however, and this started a trend toward buying "bare" disks (often by mail
order) and installing them directly into a system. Hard disk drive makers
started marketing to end users as well as OEMs, and by the mid-1990s, HDDs had
become available on retail store shelves.
While internal disks became the system of choice on PCs, external HDDs remained
popular for much longer on the Apple Macintosh and other platforms. The first
Apple Macintosh built between 1984 and 1986 had a closed architecture that did
not support an external or internal hard drive. In 1986, Apple added a SCSI port
on the back, making external expansion easy. External SCSI drives were also
popular with older microcomputers such as the Apple II series, and were also
used extensively in servers, a usage which is still popular today. The
appearance in the late 1990s of high-speed external interfaces such as USB and
FireWire has made external disk systems popular among PC users once again,
especially for laptop users, users that install Linux in the additional external
unit and users who move large amounts of data between two or more areas. Most
HDD makers now make their disks available in external cases.
Hard disk drive characteristics
5.25" MFM 110 MB HDD (2.5" ATA 6495 MB HDD, US & UK pennies for
comparison)Capacity of a hard disk drive is usually quoted in gigabytes. Older
HDDs quoted their smaller capacities in megabytes.
The data transfer rate at the inner zone ranges from 44.2 MB/s to 74.5 MB/s,
while the transfer rate at the outer zone ranges from 74.0 MB/s to 111.4 MB/s.
An HDD's random access time ranges from 5 ms to 15 ms.
The physical size of a hard disk drive is quoted in inches. The majority of HDDs
used in desktops today are 3.5" wide, while those used in laptops are 2.5" wide.
As of early 2007, manufacturers have started selling SATA and SAS 2.5 inch
drives for use in servers and desktops.
An increasingly common form factor is the 1.8" ATA-7 LIF form factor used inside
digital audio players and subnotebooks, which provide up to 160GB storage
capacity at low power consumption and are highly shock-resistant. A previous
1.8" HDD standard exists, for 2–5GB sized disks that fit directly into a PC card
expansion slot. From these, the smaller 1" form factor was evolved, which is
designed to fit the dimensions of CF Type II, which is also usually used as
storage for portable devices including digital cameras. 1" was a de facto form
factor led by IBM's Microdrive, but is now generically called 1" due to other
manufacturers producing similar products. There is also a 0.85 inch form factor
produced by Toshiba for use in mobile phones and similar applications, including
SD/MMC slot compatible HDDs optimized for video storage on 4G handsets.
The size designations are more nomenclature than descriptive. The names refer to
the width of the disk inserted into the drive rather than the actual width of
the entire drive. A 5.25" drive has an actual width of 5.75", a 3.5" drive 4", a
2.5" drive 2.75". A 1.8" drive can have different widths, depending on its form
factor. A PCMCIA drive has a width of 54 mm, while an ATA-7 LIF form factor
drive has a width of 53.85 mm.
A hard disk is defined to be at "full height" if its height is 3.25". It is
"half height" at a height of 1.625". A "slim height" or "low profile" HDD has a
height of 1". "Ultra low profile" drives can have heights of 0.75", 0.67", 0.49"
or 0.37"[citation needed].
Integrity
An IBM HDD head resting on a disk platter. Since the drive is not in operation,
the head is simply pressed against the disk by the suspension.The HDD's spindle
system relies on air pressure inside the enclosure to support the heads at their
proper flying height while the disk rotates. An HDD requires a certain range of
air pressures in order to operate properly. The connection to the external
environment and pressure occurs through a small hole in the enclosure (about 0.5
mm in diameter), usually with a carbon filter on the inside (the breather
filter, see below). If the air pressure is too low, then there is not enough
lift for the flying head, so the head gets too close to the disk, and there is a
risk of head crashes and data loss. Specially manufactured sealed and
pressurized disks are needed for reliable high-altitude operation, above about
10,000 feet (3,000 m). This does not apply to pressurized enclosures, like an
airplane pressurized cabin. Modern disks include temperature sensors and adjust
their operation to the operating environment. Breather holes can be seen on all
disks — they usually have a sticker next to them, warning the user not to cover
the holes. The air inside the operating disk is constantly moving too, being
swept in motion by friction with the spinning platters. This air passes through
an internal recirculation (or "recirc") filter to remove any leftover
contaminants from manufacture, any particles or chemicals that may have somehow
entered the enclosure, and any particles or outgassing generated internally in
normal operation. Very high humidity for extended periods can corrode the heads
and platters. If the disk uses "Contact Start/Stop" (CSS) technology to park its
heads on the platters when not operating, increased humidity can also lead to
increased stiction (the tendency for the heads to stick to the platter surface).
This can cause physical damage to the platter and spindle motor and cause head
crash.
Close-up of a hard disk head resting on a disk platter, and its suspension. A
reflection of the head and suspension are visible beneath on the mirror-like
disk.Due to the extremely close spacing between the heads and the disk surface,
any contamination of the read-write heads or platters can lead to a head crash —
a failure of the disk in which the head scrapes across the platter surface,
often grinding away the thin magnetic film. For giant magnetoresistive (GMR)
heads in particular, a minor head crash from contamination (that does not remove
the magnetic surface of the disk) still results in the head temporarily
overheating, due to friction with the disk surface, and can render the data
unreadable for a short period until the head temperature stabilizes (so called
"thermal asperity," a problem which can partially be dealt with by proper
electronic filtering of the read signal). Head crashes can be caused by
electronic failure, a sudden power failure, physical shock, wear and tear,
corrosion, or poorly manufactured platters and heads. In most desktop and server
disks, when powering down, the heads are moved to a landing zone, an area of the
platter usually near its inner diameter (ID), where no data are stored. This
area is called the CSS (Contact Start/Stop) zone. However, especially in old
models, sudden power interruptions or a power supply failure can sometimes
result in the device shutting down with the heads in the data zone, which
increases the risk of data loss. In fact, it used to be procedure to "park" the
hard disk before shutting down your computer. Newer disks are designed such that
either a spring (at first) or (more recently) rotational inertia in the platters
is used to safely park the heads in the case of unexpected power loss.
The hard disk's electronics control the movement of the actuator and the
rotation of the disk, and perform reads and writes on demand from the disk
controller. Modern disk firmware is capable of scheduling reads and writes
efficiently on the platter surfaces and remapping sectors of the media which
have failed. Also, most major hard disk and motherboard vendors now support
self-monitoring, analysis, and reporting technology (S.M.A.R.T.), which attempt
to alert users to impending failures.
However, not all failures are predictable. Normal use eventually can lead to a
breakdown in the inherently fragile device, which makes it essential for the
user to periodically back up the data onto a separate storage device. Failure to
do so can lead to the loss of data. While it may be possible to recover lost
information, it is normally an extremely costly procedure, and it is not
possible to guarantee success in the attempt. A 2007 study published by Google
suggested very little correlation between failure rates and either high
temperature or activity level. While several S.M.A.R.T. parameters have an
impact on failure probability, a large fraction of failed drives do not produce
predictive S.M.A.R.T. parameters. S.M.A.R.T. parameters alone may not be useful
for predicting individual drive failures.
Landing zones
Microphotograph of a hard disk head. The size of the front face (which is the
"trailing face" of the slider) is about 0.3 mm × 1.0 mm. The (not visible)
bottom face of the slider is about 1.0 mm × 1.25 mm (so called "nano" size) and
faces the platter. One functional part of the head is the round, orange
structure in the middle - the lithographically defined copper coil of the write
transducer. Also note the electric connections by wires bonded to gold-plated
pads.Spring tension from the head mounting constantly pushes the heads towards
the platter. While the disk is spinning, the heads are supported by an air
bearing and experience no physical contact or wear. In CSS drives the sliders
carrying the head sensors (often also just called heads) are designed to
reliably survive a number of landings and takeoffs from the media surface,
though wear and tear on these microscopic components eventually takes its toll.
The heads typically land in a "landing zone" that does not contain user data.
Most manufacturers design the sliders to survive 50,000 contact cycles before
the chance of damage on startup rises above 50%. However, the decay rate is not
linear—when a disk is younger and has fewer start-stop cycles, it has a better
chance of surviving the next startup than an older, higher-mileage disk (as the
head literally drags along the disk's surface until the air bearing is
established). For example, the Seagate Barracuda 7200.10 series of desktop hard
disks are rated to 50,000 start-stop cycles. This means that no failures
attributed to the head-platter interface were seen before at least 50,000
start-stop cycles during testing.
Around 1995 IBM pioneered a technology where a landing zone on the disk is made
by a precision laser process (Laser Zone Texture = LZT) producing an array of
smooth nanometer-scale "bumps" in a landing zone, thus vastly improving stiction
and wear performance. This technology is still largely in use today (2006). In
most mobile applications, the heads are lifted off the platters onto plastic
"ramps" near the outer disk edge, thus eliminating the risks of wear and
stiction altogether and greatly improving non-operating shock performance. All
HDDs use one of these two technologies. Each has a list of advantages and
drawbacks in terms of loss of storage space, relative difficulty of mechanical
tolerance control, cost of implementation, etc.
IBM created a technology for their ThinkPad line of laptop computers called the
Active Protection System. When a sudden, sharp movement is detected by the
built-in motion sensor in the Thinkpad, internal hard disk heads automatically
unload themselves into the parking zone to reduce the risk of any potential data
loss or scratches made. Apple later also utilized this technology in their
PowerBook, iBook, MacBook Pro, and MacBook line, known as the Sudden Motion
Sensor. Toshiba has released similar technology in their laptops.
Access and interfaces
Hard disk drives are accessed over one of a number of bus types, including ATA
(IDE, EIDE), Serial ATA (SATA), SCSI, SAS, and Fibre Channel. Bridge circuitry
is sometimes used to connect hard disk drives to busses that they cannot
communicate with natively, such as IEEE 1394 and USB.
Back in the days of the ST-506 interface, the data encoding scheme was also
important. The first ST-506 disks used Modified Frequency Modulation (MFM)
encoding, and transferred data at a rate of 5 megabits per second. Later on,
controllers using 2,7 RLL (or just "RLL") encoding increased the transfer rate
by fifty percent, to 7.5 megabits per second; it also increased disk capacity by
fifty percent.
Many ST-506 interface disk drives were only specified by the manufacturer to run
at the lower MFM data rate, while other models (usually more expensive versions
of the same basic disk drive) were specified to run at the higher RLL data rate.
In some cases, a disk drive had sufficient margin to allow the MFM specified
model to run at the faster RLL data rate; however, this was often unreliable and
was not recommended. (An RLL-certified disk drive could run on a MFM controller,
but with 1/3 less data capacity and speed.)
Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI
disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this
was usually negotiated automatically by the disk drive and controller; most of
the time, however, 15 or 20 megabit ESDI disk drives weren't downward compatible
(i.e. a 15 or 20 megabit disk drive wouldn't run on a 10 megabit controller).
ESDI disk drives typically also had jumpers to set the number of sectors per
track and (in some cases) sector size.
SCSI originally had just one speed, 5 MHz (for a maximum data rate of 5
megabytes per second), but later this was increased dramatically. The SCSI bus
speed had no bearing on the disk's internal speed because of buffering between
the SCSI bus and the disk drive's internal data bus; however, many early disk
drives had very small buffers, and thus had to be reformatted to a different
interleave (just like ST-506 disks) when used on slow computers, such as early
IBM PC compatibles and early Apple Macintoshes.
ATA disks have typically had no problems with interleave or data rate, due to
their controller design, but many early models were incompatible with each other
and couldn't run in a master/slave setup (two disks on the same cable). This was
mostly remedied by the mid-1990s, when ATA's specification was standardised and
the details began to be cleaned up, but still causes problems occasionally
(especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and
non-UDMA devices).
Serial ATA does away with master/slave setups entirely, placing each disk on its
own channel (with its own set of I/O ports) instead.
FireWire/IEEE 1394 and USB(1.0/2.0) HDDs are external units containing generally
ATA or SCSI disks with ports on the back allowing very simple and effective
expansion and mobility. Most FireWire/IEEE 1394 models are able to daisy-chain
in order to continue adding peripherals without requiring additional ports on
the computer itself.
Disk families used in personal computers
Notable disk families include:
Bit Serial Interfaces — These families connected to a hard disk drive controller
with three cables, one for data, one for control and one for power. The HDD
controller provided significant functions such as serial to parallel conversion,
data separation and track formatting, and required matching to the drive in
order to assure reliability.
ST506 used MFM (Modified Frequency Modulation) for the data encoding method.
ST412 was available in either MFM or RLL (Run Length Limited) variants.
ESDI (Enhanced Small Disk Interface) was an interface developed by Maxtor to
allow faster communication between the PC and the disk than MFM or RLL.
Word Serial Interfaces — These families connect to a host bus adapter (today
typically integrated into the "South Bridge") with two cables, one for
data/control and one for power. The earliest versions of these interfaces
typically had a 16 bit parallel data transfer to/from the drive and there are 8
and 32 bit variants. Modern versions have serial data transfer. The word nature
of data transfer makes the design of a host bus adapter significantly simpler
than that of the precursor HDD controller.
Integrated Drive Electronics (IDE) was later renamed to ATA, and then later,
PATA ("parallel ATA", to distinguish it from the new serial ATA interface,
SATA). The name comes from the way early families had the HDD controller
external to the disk. Moving the HDD controller from the interface card to the
disk helped to standardize interfaces, including reducing the cost and
complexity. The 40 pin IDE/ATA connection of PATA transfers 16 bits of data at a
time on the data cable. The data cable was originally 40 conductor, but later
higher speed requirements for data transfer to and from the hard drive led to an
"ultra DMA" mode, known as UDMA, which required an 80 conductor variant of the
same cable; the other conductors provided the grounding necessary for enhanced
high-speed signal quality. The interface for 80 pin only has 39 pins, the
missing pin acting as a key to prevent incorrect insertion of the connector to
an incompatible socket, a common cause of disk and controller damage.
EIDE was an unofficial update (by Western Digital) to the original IDE standard,
with the key improvement being the use of DMA ("Direct memory access") to
transfer data between the disk and the computer without the involvement of the
CPU, an improvement later adopted by the official ATA standards. By directly
transferring data between memory and disk, DMA does not require the
CPU/program/operating system to leave other tasks idle while the data transfer
occurs.
SCSI (Small Computer System Interface) was an early competitor with ESDI,
originally named SASI for Shugart Associates. SCSI disks were standard on
servers, workstations, and Apple Macintosh computers through the mid-90s, by
which time most models had been transitioned to IDE (and later, SATA) family
disks. Only in 2005 did the capacity of SCSI disks fall behind IDE disk
technology, though the highest-performance disks are still available in SCSI and
Fibre Channel only. The length limitations of the data cable allows for external
SCSI devices. Originally SCSI data cables used single ended data transmission,
but server class SCSI could use differential transmission, and then Fibre
Channel (FC) interface, and then more specifically the Fibre Channel Arbitrated
Loop (FC-AL), connected SCSI HDDs using fibre optics. FC-AL is the cornerstone
of storage area networks, although other protocols like iSCSI and ATA over
Ethernet have been developed as well.
SATA (Serial ATA). The SATA data cable has one data pair for differential
transmission of data to the device, and one pair for differential receiving from
the device, just like EIA-422. That requires that data be transmitted serially.
The same differential signaling system is used in RS485, LocalTalk, USB,
Firewire, and differential SCSI.
SAS (Serial Attached SCSI). The SAS is a new generation serial communication
protocol for devices designed to allow for much higher speed data transfers and
is compatible with SATA. SAS uses serial communication instead of the parallel
method found in traditional SCSI devices but still uses SCSI commands for
interacting with SAS
Acronym Meaning Description
SASI Shugart Associates System Interface Predecessor to SCSI
SCSI Small Computer System Interface Bus oriented that handles concurrent
operations.
SAS Serial Attached SCSI
ATA Advanced Technology Attachment Successor to ST-412/506/ESDI by integrating
the disk controller completely onto the device. Incapable of concurrent
operations.
SATA Serial ATA
ST-506 Seagate interface
ST-412 Seagate interface (minor improvement over ST-506)
ESDI Enhanced Small Disk Interface Faster and more integrated than ST-412/506,
but still backwards compatible
Manufacturers
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Seagate 3.5 inch 40 GB HDD.The technological resources and know-how required for
modern drive development and production mean that as of 2007, over 98% of the
world's HDDs are manufactured by just a handful of large firms: Seagate, Western
Digital, Samsung, and Hitachi (which owns the former disk manufacturing division
of IBM). Fujitsu continues to make mobile- and server-class disks but exited the
desktop-class market in 2001. Toshiba is a major manufacturer of 2.5-inch and
1.8-inch notebook disks. ExcelStor is a small HDD manufacturer.
Dozens of former HDD manufacturers have gone out of business, merged, or closed
their HDD divisions; as capacities and demand for products increased, profits
became hard to find, and the market underwent significant consolidation in the
late 1980s and late 1990s. The first notable casualty of the business in the PC
era was Computer Memories Inc. or CMI; after an incident with faulty 20 MB AT
disks in 1985, CMI's reputation never recovered, and they exited the HDD
business in 1987. Another notable failure was MiniScribe, who went bankrupt in
1990 after it was found that they had engaged in accounting fraud and inflated
sales numbers for several years. Many other smaller companies (like Kalok,
Microscience, LaPine, Areal, Priam and PrairieTek) also did not survive the
shakeout, and had disappeared by 1993; Micropolis was able to hold on until
1997, and JTS, a relative latecomer to the scene, lasted only a few years and
was gone by 1999, after attempting to manufacture HDDs in India. Their claim to
fame was creating a new 3" form factor drive for use in laptops. Quantum and
Integral also invested in the 3" form factor; but eventually gave up as this
form factor failed to catch on.[citation needed] Rodime was also an important
manufacturer during the 1980s, but stopped making disks in the early 1990s amid
the shakeout and now concentrates on technology licensing; they hold a number of
patents related to 3.5-inch form factor HDDs.
This list is incomplete; you can help by expanding it.
1988: Tandem Computers sold its disk manufacturing division to Western Digital
(WDC), which was then a well-known controller designer.
1989: Seagate Technology bought Control Data's high-end disk business, as part
of CDC's exit from hardware manufacturing.
1990: Maxtor buys MiniScribe out of bankruptcy, making it the core of its
low-end disk division.
1994: Quantum bought DEC's storage division, giving it a high-end disk range to
go with its more consumer-oriented ProDrive range, as well as the DLT tape drive
range.
1995: Conner Peripherals, which was founded by one of Seagate Technology's
co-founders along with personnel from MiniScribe, announces a merger with
Seagate, which was completed in early 1996.
1996: JTS merges with Atari, allowing JTS to bring its disk range into
production. Atari was sold to Hasbro in 1998, while JTS itself went bankrupt in
1999.
2000: Quantum sells its disk division to Maxtor to concentrate on tape drives
and backup equipment.
2003: Following the controversy over mass failures of its Deskstar 75GXP range,
HDD pioneer IBM sold the majority of its disk division to Hitachi, who renamed
it Hitachi Global Storage Technologies (HGST).
December 21, 2005: Seagate and Maxtor announced an agreement under which Seagate
would acquire Maxtor in an all stock transaction valued at $1.9 billion. The
acquisition was approved by the appropriate regulatory bodies, and closed on May
19, 2006.
2007: Hitachi releases the 1TB (1024 Gigabytes (GB) = 1 Terabyte (TB) ) Hitachi
Deskstar 7k100.
2007: Western Digital (WDC) acquire Komag U.S.A, a thin-film media manufacturer,
for USD 1 Billion.
See also
Click of death
Data recovery
Disk Usage
Disk formatting
Hybrid drive
Native command queuing
PRML
Solid state drive
Superparamagnetism
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