">![]() Video of modern HDD operation (cover removed)
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Date invented | 24 December 1954[note 1] |
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Invented by | IBM team led by Rey Johnson |
A hard disk drive (HDD)[note 2] is a data storage device used for storing and retrieving digital information using rapidly rotating discs (platters) coated with magnetic material. An HDD retains its data even when powered off. Data is read in a random-access manner, meaning individual blocks of data can be stored or retrieved in any order rather than just sequentially. An HDD consists of one or more rigid ("hard") rapidly rotating discs (platters) with magnetic heads arranged on a moving actuator arm to read and write data to the surfaces.
Introduced by IBM in 1956,[1] HDDs became the dominant secondary storage device for general purpose computers by the early 1960s. Continuously improved, HDDs have maintained this position into the modern era of servers and PCs. More than 200 companies have produced HDD units, though most current units are manufactured by Western Digital, Seagate, and Toshiba. Worldwide revenues for HDDs shipments are expected to reach $38 billion in 2012, up about 19% from $32 billion in 2011.
The primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabytes (GB; where 1 gigabyte = 1 billion bytes). Typically, some of an HDD's capacity is unavailable to the user due to use by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. Performance is specified by the time to move the heads to a file (average access time) plus the time it takes for the file to move under its head (average latency, a function of the physical rotational speed in revolutions per minute) and the speed at which the file is transmitted (data rate).
The two most common form factors for modern HDDs are 3.5-inch in desktop computers and 2.5-inch in laptops. Different sizes are used in specialty devices such as portable media players or in some server hardware. HDDs are connected to systems by standard interface cables such as SATA (Serial ATA), USB or SAS (Serial attached SCSI) cables.
As of 2012[update], the primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs). HDDs are expected to remain the dominant medium for secondary storage for the forseeable future due to advantages in recording capacity and price per unit of storage,[2][3] but solid state drives are replacing rotating hard drives especially in portable electronics where speed, physical size, and durability are more important considerations than price and capacity.[4][5]
HDDs were introduced in 1956 as data storage for an IBM real-time transaction processing computer[1] and were developed for use with general purpose mainframe and minicomputers. The first IBM drive, the 350 RAMAC, was approximately the size of two refrigerators and stored 5 million 6-bit characters (the equivalent of 3.75 million 8-bit bytes) on a stack of 50 discs.
In 1961 IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were called fixed disk drives.
In 1973, IBM introduced a new type of HDD codenamed "Winchester". Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to "land" on a special area of the disk surface upon spin-down, "taking off" again when the disk was later powered on. This greatly reduced the cost of the head actuator mechanism, but precluded removing just the disks from the drive as was done with the disk packs of the day. Instead, the first models of "Winchester technology" drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal. Later "Winchester" drives abandoned the removable media concept and returned to non-removable platters.
Like the first removable pack drive, the first "Winchester" drives used platters 14 inches (360 mm) in diameter. A few years later, designers were exploring the possibility that physically smaller platters might offer advantages. Drives with non-removable eight-inch platters appeared, and then drives that used a [Convert: Invalid number] form factor (a mounting width equivalent to that used by contemporary floppy disk drives). The latter were primarily intended for the then-fledgling personal computer (PC) market.
As the 1980s began, HDDs were a rare and very expensive additional feature on PCs; however by the late 1980s, their cost had been reduced to the point where they were standard on all but the cheapest PC.
Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. The subsystem was not sold under the drive manufacturer's name but under the subsystem manufacturer's name such as Corvus Systems and Tallgrass Technologies, or under the PC system manufacturer's name such as the Apple ProFile. The IBM PC/XT in 1983 included an internal 10MB HDD, and soon thereafter internal HDDs proliferated on personal computers.
External HDDs remained popular for much longer on the Apple Macintosh. Every Mac made between 1986 and 1998 has a SCSI port on the back, making external expansion easy; also, "toaster" Compact Macs did not have easily accessible HDD bays (or, in the case of the Mac Plus, any hard drive bay at all), so on those models, external SCSI disks were the only reasonable option.
Driven by areal density doubling every two to four years since their invention (an observation known as Kryder's law, not unlike Moore's Law), HDDs have continuously improved their characteristics; a few highlights include:
An HDD records data by magnetizing a thin film of ferromagnetic material on a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization. User data is encoded using an encoding scheme, such as run-length limited encoding,[9] which determines how the data is represented by the magnetic transitions.
A typical HDD design consists of a spindle that holds flat circular disks, also called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminium alloy, glass, or ceramic, and are coated with a shallow layer of magnetic material typically 10?20 nm in depth, with an outer layer of carbon for protection.[10][11][12] For reference, a standard piece of copy paper is 0.07–0.18 millimetres (70,000–180,000 nm).[13]
The platters in contemporary HDDs are spun at speeds varying from 4,200 rpm in energy-efficient portable devices, to 15,000 rpm for high performance servers.[15] The first HDDs spun at 1,200 rpm[16] and, for many years, 3,600 rpm was the norm.[17] Today, most consumer HDDs operate at a speed of 7,200 rpm.
Information is written to and read from a platter as it rotates past devices called read-and-write heads that operate very close (often tens of nanometers) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. In modern drives there is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or in some older designs a stepper motor.
In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.[18] Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, first shipped in 2005,[19] and as of 2007 the technology was used in many HDDs.[20][21][22]
A typical HDD has two electric motors; a disk motor that spins the disks and an actuator (motor) that positions the read/write head assembly across the spinning disks. The disk motor has an external rotor attached to the disks; the stator windings are fixed in place. Opposite the actuator at the end of the head support arm is the read-write head; thin printed-circuit cables connect the read-write heads to amplifier electronics mounted at the pivot of the actuator. The head support arm is very light, but also stiff; in modern drives, acceleration at the head reaches 550 g.
The actuator is a permanent magnet and moving coil motor that swings the heads to the desired position. A metal plate supports a squat neodymium-iron-boron (NIB) high-flux magnet. Beneath this plate is the moving coil, often referred to as the voice coil by analogy to the coil in loudspeakers, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor (some drives only have one magnet).
The voice coil itself is shaped rather like an arrowhead, and made of doubly coated copper magnet wire. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it is wound on a form, making it self-supporting. The portions of the coil along the two sides of the arrowhead (which point to the actuator bearing center) interact with the magnetic field, developing a tangential force that rotates the actuator. Current flowing radially outward along one side of the arrowhead and radially inward on the other produces the tangential force. If the magnetic field were uniform, each side would generate opposing forces that would cancel each other out. Therefore the surface of the magnet is half N pole, half S pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling. Currents along the top and bottom of the coil produce radial forces that do not rotate the head.
The HDD'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. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback. These are either complete concentric circles (in the case of dedicated servo technology), or segments interspersed with real data (in the case of embedded servo technology). The servo feedback optimizes the signal to noise ratio of the GMR sensors by adjusting the voice-coil of the actuated arm. The spinning of the disk also uses a servo motor. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.
Modern drives make extensive use of error correction codes (ECCs), particularly Reed?Solomon error correction. These techniques store extra bits, determined by mathematical formulas, for each block of data; the extra bits allow many errors to be corrected invisibly. The extra bits themselves take up space on the HDD, but allow higher recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity.[23] In the newest drives of 2009, low-density parity-check codes (LDPC) were supplanting Reed-Solomon; LDPC codes enable performance close to the Shannon Limit and thus provide the highest storage density available.[24]
Typical HDDs attempt to "remap" the data in a physical sector that is failing to a spare physical sector?hopefully while the errors in the bad sector are still few enough that the ECC can recover the data without loss. The S.M.A.R.T-Self-Monitoring, Analysis and Reporting Technology system counts the total number of errors in the entire HDD fixed by ECC and the total number of remappings, as the occurrence of many such errors may predict HDD failure.
HDD areal densities have shown a long term compound annual growth rate not substantively different from Moore's Law, most recently in the range of 20-25% annually.[25] New magnetic storage technologies are being developed to support higher areal density growth and maintain the competitiveness of HDDs with potentially competitive products such as Flash based SSDs (Solid State Drives). These new HDD technologies include:
With these new technologies the relative position of HDDs and SSDs with regard to their cost and performance is not projected to change through 2016.[25]
The capacity of an HDD reported to an end user by the operating system is less than the amount stated by a drive or system manufacturer due to amongst other things, different units of measuring capacity, capacity consumed by the file system and/or redundancy.
Because modern disk drives appear to their interface as a contiguous set of logical blocks their gross capacity can be calculated by multiplying the number of blocks by the size of the block. This information is available from the manufacturers specification and from the drive itself through use of special utilities invoking low level commands[29][30]
The gross capacity of older HDDs can be calculated by multiplying for each zone of the drive the number of cylinders by the number of heads by the number of sectors/zone by the number of bytes/sector (most commonly 512) and then summing the totals for all zones. Some modern SATA drives will also report cylinder-head-sector (C/H/S) values to the CPU but they are no longer actual physical parameters since the reported numbers are constrained by historic operating-system interfaces.
The old C/H/S scheme has been replaced by logical block addressing. In some cases, to try to "force-fit" the C/H/S scheme to large-capacity drives, the number of heads was given as 64, although no modern drive has anywhere near 32 platters.
In modern HDDs spare capacity for defect management is not included in the published capacity; however in many early HDDs a certain number of sectors were reserved for spares, thereby reducing capacity available to end users.
In some systems, there may be hidden partitions used for system recovery that reduce the capacity available to the end user.
For RAID subsystems, data integrity and fault-tolerance requirements also reduce the realized capacity. For example, a RAID1 subsystem will be about half the total capacity as a result of data mirroring. RAID5 subsystems with x drives, would lose 1/x of capacity to parity. RAID subsystems are multiple drives that appear to be one drive or more drives to the user, but provides a great deal of fault-tolerance. Most RAID vendors use some form of checksums to improve data integrity at the block level. For many vendors, this involves using HDDs with sectors of 520 bytes per sector to contain 512 bytes of user data and 8 checksum bytes or using separate 512 byte sectors for the checksum data.[31]
The presentation of an HDD to its host is determined by its controller. This may differ substantially from the drive's native interface particularly in mainframes or servers.
Modern HDDs, such as SAS[29] and SATA[30] drives, appear at their interfaces as a contiguous set of logical blocks; typically 512 bytes long but the industry is in the process of changing to 4,096 byte logical blocks; see Advanced Format.[32]
The process of initializing these logical blocks on the physical disk platters is called low level formatting which is usually performed at the factory and is not normally changed in the field.[note 3]
High level formatting then writes the file system structures into selected logical blocks to make the remaining logical blocks available to the host OS and its applications.[33] The operating system file system uses some of the disk space to organize files on the disk, recording their file names and the sequence of disk areas that represent the file. Examples of data structures stored on disk to retrieve files include the file allocation table (FAT) in MS DOS and inodes in UNIX, as well as other operating system data structures. As a consequence not all the space on a HDD is available for user files. This file system overhead is usually less than 1% on drives larger than 100 MB.
Unit prefixes[34][35] | Advertised capacity by manufacturer (using decimal?multiples) | Expected capacity by consumers in class action (using binary?multiples) | Reported capacity | |||
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Windows (using binary multiples) | Mac OS X 10.6 (using decimal multiples) | |||||
With prefix | Bytes | Bytes | Diff. | |||
100?MB | 100,000,000 | 104,857,600 | 4.86% | 95.4?MB | 100?MB | |
100?GB | 100,000,000,000 | 107,374,182,400 | 7.37% | 93.1?GB, 95,367?MB | 100 GB | |
1?TB | 1,000,000,000,000 | 1,099,511,627,776 | 9.95% | 931?GB, 953,674?MB | 1,000?GB, 1,000,000?MB |
The total capacity of HDDs is given by manufacturers in megabytes (1?MB?=?1,000,000 bytes), gigabytes (1?GB?=?1,000,000,000 bytes) or terabytes (1?TB?=?1,000,000,000,000 bytes).[36][37][38][39][40][41] This numbering convention, where prefixes like mega- and giga- denote powers of 1,000, is also used for data transmission rates and DVD capacities. However, the convention is different from that used by manufacturers of memory (RAM, ROM) and CDs, where prefixes like kilo- and mega- mean powers of 1,024.
The practice of using prefixes assigned to powers of 1,000 within the HDD and computer industries dates back to the early days of computing.[42] By the 1970s million, mega and M were consistently being used in the powers of 1,000 sense to describe HDD capacity.[43][44][45]
Computers do not internally represent HDD or memory capacity in powers of 1,024; reporting it in this manner is just a convention.[46] Microsoft Windows uses the powers of 1,024 convention when reporting HDD capacity, thus an HDD offered by its manufacturer as a 1 TB drive is reported by these OSes as a 931 GB HDD. Mac OS?X 10.6 ("Snow Leopard"), uses powers of 1,000 when reporting HDD capacity.
In the case of "mega-", there is a nearly 5% difference between the powers of 1,000 definition and the powers of 1,024 definition. Furthermore, the difference is compounded by 2.4% with each incrementally larger prefix (gigabyte, terabyte, etc.) The discrepancy between the two conventions for measuring capacity was the subject of several class action suits against HDD manufacturers. The plaintiffs argued that the use of decimal measurements effectively misled consumers[47][48] while the defendants denied any wrongdoing or liability, asserting that their marketing and advertising complied in all respects with the law and that no class member sustained any damages or injuries.[49]
In December 1998, an international standards organization attempted to address these dual definitions of the conventional prefixes by proposing unique binary prefixes and prefix symbols to denote multiples of 1,024, such as "mebibyte (MiB)", which exclusively denotes 220 or 1,048,576 bytes.[50] The proposal has seen little adoption by the computer industry, and the conventionally prefixed forms of "byte" continue to denote slightly different values depending on context.[51][52]
Past and present HDD form factors | ||||||
Form factor | Status | Width (mm) | Height (mm) | Largest capacity | Platters (max) | Capacity Per platter (GB) |
---|---|---|---|---|---|---|
3.5" | Current | 102 | 19 or 25.4 | 4 TB[53][54] (2011) | 5 | 1000 |
2.5" | Current | 69.9 | 5,[55] 7, 9.5,[note 4] 12.5, or 15 | 2?TB[56][note 5] (2012) | 4 | 500 |
1.8" | Current | 54 | 5 or 8 | 320?GB[57] (2009) | 2 | 160 |
5.25" FH | Obsolete | 146 | 47?GB[58] (1998) | 14 | 3.36 | |
5.25" HH | Obsolete | 146 | 19.3?GB[59] (1998) | 4[note 6] | 4.83 | |
1.3" | Obsolete | 43 | 40?GB[60] (2007) | 1 | 40 | |
1" (CFII/ZIF/IDE-Flex) | Obsolete | 42 | 20?GB (2006) | 1 | 20 | |
0.85" | Obsolete | 24 | 8?GB[61][62] (2004) | 1 | 8 |
Mainframe and minicomputer hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machines or designed to fit a 19" rack. In 1962, IBM introduced its model 1311 disk, which used 14?inch (nominal size) platters. This became a standard size for mainframe and minicomputer drives for many years,[63] but such large platters were never used with microprocessor-based systems.
With increasing sales of microcomputers having built in floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable. Thus HDD Form factors, initially followed those of 8-inch, 5.25-inch, and 3.5-inch floppy disk drives. Because there were no smaller floppy disk drives, smaller HDD form factors developed from product offerings or industry standards.
As of 2012[update], 3.5-inch and 2.5-inch hard disks were the most popular sizes.
By 2009 all manufacturers had discontinued the development of new products for the 1.3-inch, 1-inch and 0.85-inch form factors due to falling prices of flash memory,[76][77] which has no moving parts.
While these sizes are customarily described by an approximately correct figure in inches, actual sizes have long been specified in millimeters.
The factors that limit the time to access the data on an HDD (Access time) are mostly related to the mechanical nature of the rotating disks and moving heads. Seek time is a measure of how long it takes the head assembly to travel to the track of the disk that contains data. Rotational latency is incurred because the desired disk sector may not be directly under the head when data transfer is requested. These two delays are on the order of milliseconds each. The bit rate or data transfer rate (once the head is in the right position) creates delay which is a function of the number of blocks transferred; typically relatively small, but can be quite long with the transfer of large contiguous files. Delay may also occur if the drive disks are stopped to save energy.
An HDD's Average Access Time is its average Seek time which technically is the time to do all possible seeks divided by the number of all possible seeks, but in practice is determined by statistical methods or simply approximated as the time of a seek over one-third of the number of tracks[78]
Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk.[79] Some computer operating systems perform defragmentation automatically. Although automatic defragmentation is intended to reduce access delays, performance will be temporarily reduced while the procedure is in progress.[80]
Access time can be improved by increasing rotational speed (thus reducing latency) and/or by reducing the time spent seeking. Increasing areal density increases throughput by increasing data rate and by increasing the amount of data under a set of heads, thereby potentially reducing seek activity for a given amount of data. Based on historic trends, analysts predict a future growth in HDD areal density (and therefore capacity) of about 40% per year.[81] Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.
Average seek time ranges from 3?ms[82] for high-end server drives, to 15?ms for mobile drives, with the most common mobile drives at about 12?ms[83] and the most common desktop type typically being around 9?ms. The first HDD had an average seek time of about 600 ms;[84] by the middle 1970s HDDs were available with seek times of about 25 ms.[85] Some early PC drives used a stepper motor to move the heads, and as a result had seek times as slow as 80?120?ms, but this was quickly improved by voice coil type actuation in the 1980s, reducing seek times to around 20?ms. Seek time has continued to improve slowly over time.
Some desktop and laptop computer systems allow the user to make a tradeoff between seek performance and drive noise. Faster seek rates typically require more energy usage to quickly move the heads across the platter, causing loud noises from the pivot bearing and greater device vibrations as the heads are rapidly accelerated during the start of the seek motion and decelerated at the end of the seek motion. Quiet operation reduces movement speed and acceleration rates, but at a cost of reduced seek performance.
Latency is the delay for the rotation of the disk to bring the required disk sector under the read-write mechanism. It depends on rotational speed of a disk, measured in revolutions per minute (rpm). Average rotational latency is shown in the table below, based on the statistical relation that the average latency in milliseconds for such a drive is one-half the rotational period.
Rotational speed (rpm) | Average latency (ms) |
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15,000 | 2 |
10,000 | 3 |
7,200 | 4.16 |
5,400 | 5.55 |
4,800 | 6.25 |
As of 2010[update], a typical 7,200 rpm desktop HDD has a sustained "disk-to-buffer" data transfer rate up to 1,030 Mbits/sec.[86] This rate depends on the track location; the rate is higher for data on the outer tracks (where there are more data sectors per rotation) and lower toward the inner tracks (where there are fewer data sectors per rotation); and is generally somewhat higher for 10,000 rpm drives. A current widely used standard for the "buffer-to-computer" interface is 3.0 Gbit/s SATA, which can send about 300 megabyte/s (10-bit encoding) from the buffer to the computer, and thus is still comfortably ahead of today's disk-to-buffer transfer rates. Data transfer rate (read/write) can be measured by writing a large file to disk using special file generator tools, then reading back the file. Transfer rate can be influenced by file system fragmentation and the layout of the files.[79]
HDD data transfer rate depends upon the rotational speed of the platters and the data recording density. Because heat and vibration limit rotational speed, advancing density becomes the main method to improve sequential transfer rates. Higher speeds require more power absorbed by the electric engine, which hence warms up more; high speeds also amplify vibrations due to baricenter of disk not being exactly in the center of the disk itself. While areal density advances by increasing both the number of tracks across the disk and the number of sectors per track, only the latter increases the data transfer rate for a given rpm. Since data transfer rate performance only tracks one of the two components of areal density, its performance improves at a lower rate.[citation needed]
Other performance considerations include power consumption, audible noise, and shock resistance.
HDDs are accessed over one of a number of bus types, including as of 2011[update] parallel ATA (PATA, also called IDE or EIDE; described before the introduction of SATA as ATA), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect HDDs to buses with which they cannot communicate natively, such as IEEE?1394, USB and SCSI.
Modern HDDs present a consistent interface to the rest of the computer, no matter what data encoding scheme is used internally. Typically a DSP in the electronics inside the HDD takes the raw analog voltages from the read head and uses PRML and Reed?Solomon error correction[87] to decode the sector boundaries and sector data, then sends that data out the standard interface. That DSP also watches the error rate detected by error detection and correction, and performs bad sector remapping, data collection for Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks.
Modern interfaces connect an HDD to a host bus interface adapter (today typically integrated into the "south bridge") with one data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit.
Due to the extremely close spacing between the heads and the disk surface, HDDs are vulnerable to being damaged by a head crash?a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, contamination of the drive's internal enclosure, wear and tear, corrosion, or poorly manufactured platters and heads.
The HDD's spindle system relies on air pressure inside the disk enclosure to support the heads at their proper flying height while the disk rotates. HDDs require 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 breadth), usually with a filter on the inside (the breather filter).[88] 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 3,000 m (9,800 ft).[89] Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on all disk drives?they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating drive 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.
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).
When a mechanical hard disk requires repairs, the easiest method is to replace the circuit board using an identical hard disk, provided it is the circuit board that has malfunctioned. In the case of read-write head faults, they can be replaced using specialized tools in a dust-free environment. If the disk platters are undamaged, they can be transferred into an identical enclosure and the data can be copied or cloned onto a new drive. In the event of disk-platter failures, disassembly and imaging of the disk platters may be required.[90] For logical damage to file systems, a variety of tools, including fsck on UNIX-like systems and CHKDSK on Windows, can be used for data recovery. Recovery from logical damage can require file carving.[91]
External removable HDDs[92] typically connect via USB. Plug and play drive functionality offers system compatibility, and features large storage options and portable design. External HDDs are available in 2.5" and 3.5" sizes, and as of March 2012[update] their capacities generally range from 160GB to 2TB. Common sizes are 160GB, 250GB, 320GB, 500GB, 640GB, 750GB, 1TB, and 2TB.[93][94]
External HDDs are available as preassembled integrated products, or may be assembled by combining an external enclosure (with USB or other interface) with a separately purchased drive.
Features such as biometric security or multiple interfaces are available at a higher cost.[95]
The exponential increases in disk space and data access speeds of HDDs have enabled consumer products that require large storage capacities, such as digital video recorders and digital audio players.[98] In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based services with extraordinary capacity requirements, such as free-of-charge web search, web archiving, and video sharing (Google, Internet Archive, YouTube, etc.).
More than 200 companies have manufactured HDDs over time. But recent consolidations have concentrated production into just three manufacturers: Western Digital, Seagate, and Toshiba.
Worldwide revenues for HDDs shipments are expected to reach $38 billion in 2012, up about 19% from $32 billion in 2011. This corresponds to a 2012 unit shipment forecast of 673 million units compared to 624 million units in 2011 and 654 million units in 2010 (the drop in 2011 was due to the impact of Thailand flooding on HDD production capacity in late 2011). The estimated 2012 market shares are about 40% each for Seagate and Western Digital and 15-20% for Toshiba.[99]
HDDs are traditionally symbolized as a stylized stack of platters or as a cylinder and are found in diagrams or on lights to indicate HDD access. In most modern operating systems, HDDs are represented by an illustration or photograph of the drive enclosure.
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