The HDD is composed of relatively inflexible aluminum, glass platters, or disks. This inflexibility led to the name hard disk drive. The hard drive is typically not removable, which is why IBM has referred to hard drives as fixed disk drives. In short, a hard disk drive is a high-volume disk storage device with fixed, high density, rigid media.

HDD Components
All hard disk drives share a common set of components. These components include: Disk platters, read/write heads, head actuator assembly, spindle motor, logic/circuit board, bezel/faceplate, configuration jumpers, and interface connectors.

Disk platters are the actual media on which data is stored in the hard disk drive. A hard disk drive typically has two to ten platters. They are usually either 2 ½" or 3 ½" in diameter and are typically constructed of aluminum or a glass-ceramic composite material. They are coated with a thin-film media that is magnetically sensitive. The platters are double-sided, with the magnetically sensitive media on each side. Platters are stacked with spaces between them on a hub that holds them in position, separate from one another. The hub is also called the spindle.

The disk platters require a read/write head for each side. The read/write head is used to access the media. The read/write heads are stacked, or ganged, on a carrier called a rack. Since they are mounted together, they move in unison across the platters with the rack. The heads are joined to the rack by arms. The arms extend from the head actuator assembly. The head itself is a U- or V-shaped device of electrically conductive material wrapped with wires, which cause it to be sensitive to the magnetic media of the platters.

While the read/write heads on floppy drives directly contact the media surface, those of hard disk drives "float" a small distance above the surface. This is because the platters typically spin at very high speeds such as 4,500-10,000 rpm (revolutions per minute), causing air pressure to build between them and the read/write head. The central hub (or spindle) on which the platters are mounted is spun by a spindle motor, which is connected to the spindle by gears. The head actuator assembly is moved across the surface of the platters and positioned by a voice coil actuator. The movement positions it over a specific track.

How the Drive Functions
The hard disk drive functions in much the same way as a floppy disk drive. The disk platters spin at a high speed while the drive heads access the media to conduct read or write operations. Understanding how the heads read and write the data structures on the platter media is critical to knowing how the drive functions.

The media on the drive platters is a layer of magnetically sensitive material. In general, modern hard disk drives use a thin-film of a cobalt metal alloy laid down in several micro-thin layers. The magnetic particles in this media are randomly aligned when the disk is empty of data. However, as the read/write head writes to an area, it causes the particles on that track to align in a specific direction. This is done according to the direction of electric current flow in the heads. This local magnetic direction in the media is referred to as a flux. The current in the head can be reversed, causing a flux reversal (opposite magnetic orientation in the media). As the platter spins, the head will lay down a flux pattern along the length of a track. This pattern of flux transitions on the track represents the recorded data.

Evolution of the Hard Drives and Drive Capacity
Personal computers have at least one HDD installed inside the system unit. If more storage capacity is needed, another HDD can usually be added. The capacity of the HDD is a measure of how much information it can store. The capacity of an HDD is normally measured in megabytes or gigabytes. Older hard disks retained about 5 MB and used platters up to 12" in diameter. Today, hard disks generally use 3.5" platters for desktop computers and 2.5" platters for notebooks. They can hold several gigabytes. A 2 GB (gigabyte) HDD, for example, can store close to 2,147,483,648 characters. This may sound large, but for today's applications and operating systems such as Windows 95, 98, Me, and 2000, 2 GB can be used up rather quickly, leaving little space for data storage purposes.

Some of the older hard disk interfaces used a device-level interface. These older hard disks had many problems with compatibility, data integrity, and speed. The original hard disk interface used in the IBM PC/XT was developed by Seagate Technologies and referred to as Modified Frequency Modulation (MFM). MFM used a magnetic disk encoding method with the ST-506 interface.

Run Length Limited (RLL) is a hard disk interface that is similar to the MFM, but has a larger number of sectors. RLL is an encoding method commonly used on magnetic disks, including RLL, SCSI, IDE, and ESDI interfaces. Currently the most common hard disk drive standards are IDE, EIDE, and SCSI.

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