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Building your PC:
The Hard Disk Drive
Introduction
A hard disk drive (also known as HDD or hard drive) is the primary data storage and retrieval system for almost all modern PCs and laptops. Physically, it is a non-volatile data storage device that stores data on a magnetic surface layered onto hard disk platters.
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| How Hard Disks Work |
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History:
The first Hard Drive was introduced in 1955 with the IBM 305 computer. This drive had fifty 24" platters and a total capacity of five million characters. It also had only a single head to access each platter so that average access time was rather slow. However, in the next generation of drives, the IBM 1301 Disk Storage Unit, announced in 1961, each platter had its own head.
Then, in 1973, IBM introduced the 3340 "Winchester" disk system (pictured above), which was the first to use a sealed head/disk assembly (HDA) which almost all modern disks now use. The term 'winchester' became almost ubiquitous in terms of describing hard disks during the 1980s, though the term gradually fell into disuse during the 1990s. Project head designer/lead designer Kenneth Haughton named it after the Winchester 30-30 rifle after the developers called it the "30-30" because of its two 30 MB spindles.
For many years, though, hard disks 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 1980s most hard disks employed either 8" or 14" platters with the drive units themselves being commensurately sized and required a large amount of floor space for housing and in many cases needed high-amperage or even three-phase power hookups due to the large motors they used.
It was not until the early 1980s that Seagate Technology introduced the ST-506, the first 5.25-inch hard drive, with a capacity of 5 megabytes. Many PCs of the time (including the original IBM 5150) were not equipped with a hard drive and most manufacturers sold these devices to OEMs for inclusion into other peripherals. However, the IBM PC/XT did have a hard drive and this started a trend for users to buy 'bare' drives for direct installation into their system. As a result hard disk manufacturers started marketing directly to end users and by the mid 1990s hard disks had become an essentially ubiquitous component.
While internal drives became the system of choice on PCs, external hard drives remained popular for much longer on the Apple Macintosh and other platforms. Every Mac made between 1986 and 1998 has a SCSI port on the back, making external expansion easy. This trend has continued to this day with the appearance of external interfaces such as USB and FireWire making the addition of external storage by regular users popular once again. With image files weighing-in at multi-gigabytes and external drives topping 200Gb in capacity being common external storage is becoming a convenient way of transferring files between machines.
The first internal PC drives were of a few megabytes in capacity (4 or 5 being usual) and in the early personal computers a drive with a 20 megabyte capacity was considered large. By the mid 1990s drives with gigabyte capacity were starting to become available and by the early 2000s drives of 20 GB were common. As of early 2006, the smallest commonly available internal PC drive stands at 40 GB with the largest-capacity internal drives being around half a terabyte (500GB) in size. External drives may be up to a terabyte in capacity and chained together can provide cheap multi-terabyte storage even for the home user.
The advent of laptops in the late 1980s necessitated a shrinkage of the hard drive so that 3.25-inch small format drives became common (though these remained small, starting at 540MB and only increasing to 4 to 8GB by the mid 1990s. Today, most laptops have hard disks of 40GB and it is not uncommon to have a 100GB internal hard drive in a laptop.
The advent of external cases for FireWire and USB connectivity has also made it possible to increase the capacity of an internal hard drive whilst still allowing the old drive to be used as an external storage device.
How Hard Drives Work
In essence a hard disk is composed of a stack of platters (disks) each with its own read-write head to access and store data. The platter itself is rigid and made from glass of aluminium and is coated on each side with a thin layer of magnetic material (usually iron(III) oxide) on which the digital data is stored.
Above each disk lies a read–write head (see image left) that is kept very close to the magnetic material of the disk itself. Information is written to the disk by transmitting an electromagnetic flux through a read-write head which in turn changes its polarization due to the flux. The information can be read by a read-write head which senses electrical change as the magnetic fields pass by in close proximity as the platter rotates.
In a typical hard disk the platters are held on a central axis or spindle that keeps the disks themselves rotating at a constant angular (rotational) velocity. Moving along and between the platters on a common armature are read-write heads, with one head for each platter surface. The armature moves the heads radially across the platters as they spin, allowing each head access to the entirety of the platter.
The associated electronics control the movement of the read-write armature and the rotation of the disk, and perform reads and writes on demand from the disk controller. Modern drive firmware is capable of scheduling reads and writes efficiently on the disk surfaces and remapping sectors of the disk which have failed. In modern drives this is in conjunction to SMART technology (by which impending failures can often be predicted, allowing the user to be alerted in time to prevent data loss).
Despite what most people think, the drive unit itself is not entirely closed and it does not hold a vacuum inside. Rather, Instead, the system relies on air pressure inside the drive to support the heads at their proper flying height while the disk is in motion. The drive also has a permeable filter lying between the top cover and inside of the drive, which allows the pressure inside and outside the drive to equalize whilst excluding dust and dirt. Unfortunately, the filter will also allow moisture in the air to enter the drive so that using drives constantly in conditions of high humidity will cause accelerated wear of the drive as the surface tension of the elevated moisture increases the tendency for the heads to stick to the disk surface, which causes physical damage to the disk and spindle motor.
One of the more common causes of hard disk failure is a 'head crash'. This is a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film. This is generally a problem due to the extremely close spacing between the heads and the surface of the disk and can be caused by residual contamination within the drive itself, electronic failure, a sudden power failure, physical shock, wear and tear, or poorly manufactured disks.
Spring tension from the head mounting constantly pushes the heads towards the disk. While the disk is spinning, the heads are supported by an air bearing and experience no physical contact wear. The sliders (the part of the heads that are closest to the disk and contain the pickup coil itself) are designed to reliably survive a number of landings and takeoffs from the disk surface, though wear and tear on these microscopic components eventually takes its toll. 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 drive is younger and has fewer start/stop cycles, it has a better chance of surviving the next startup than an older, higher-mileage drive (as the head literally drags along the drive's surface until the air bearing is established). For example, the Maxtor DiamondMax series of desktop hard drives are rated to 50,000 start-stop cycles. This means that no failures attributed to the head-disk interface were seen before at least 50,000 start-stop cycles during testing.
Hard Disk Interfaces
For a general hard drive for your system you will generally connect via one of the interfaces available build in to the motherboard which typically means an IDE (or more recently EIDE). Though more recently SATA (serial ATA) connections are now becoming commonplace and may have some advantages over traditional IDE (an SATA disk is shown left [top] with the IDE drive depicted below). Unlike IDE drives the SATA does away with the master/slave setup entirely, placing each drive on its own channel (with its own set of I/O ports) instead. This does away with the problems sometimes encountered when mixing Ultra DMA and non-UDMA devices on the same IDE chain (such as DVD drives and hard drives).
Other disk connector types include SCSI (small computer systems interface) which has faster data throughput because of buffering between the SCSI bus and the drive's internal data bus though these are controlled by additional interface cards and are generally used for high-end machines only. There is also Fibre Channel, which is a gigabit speed network technology primarily used for large scale storage networking. Disks and interfaces are much more costly than would normally be used for home PCs. There are also USB and FireWire (IEEE 1394) drives. These, however, are generally IDE drives that are placed in a housing that supports the USB or the FireWire data transfer protocol.
