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Last Updated: Monday, 21 May 2007, 14:54 GMT 15:54 UK
Factfile: Hard disk drive
A hard disk drive is the device used to store large amounts of digital information in computers and related equipment like iPods and games consoles such as the Xbox 360 and PS3.

A typical hard disk drive consists of a motor, spindle, platters, read/write heads, actuator and electronics. Click on the links below to see how these components work.

PlattersSpindleHead armRead/write headVoice coil actuatorAir filter


A hard disk has one or more platters - or disks - and each platter usually has a head on each of its sides.

In modern drives the platters are made from glass or ceramic. The platters themselves are non-magnetic but have a magnetic coating which holds the magnetic impulses which represent the data.

A typical hard drive will have three or four platters and modern varieties can hold 20GB per disk.


The read/write heads float on a cushion of air only nanometers above the surface of the platters.

As the read/write heads pass over the spinning platters they magnetize the surface in a pattern which represents the data in digital form.

The data is stored digitally as tiny magnetized regions, called bits, on the disk.

A magnetic orientation in one direction on the disk could represent a "1", while an orientation in the opposite direction could represent a "0".

Data is arranged in sectors along a number of concentric tracks. These tracks are arranged from the inner diameter of the disk to its outer edge.

When reading data on a disk, a similar process occurs in reverse.


The platters are mounted on the spindle which is turned by the drive motor. Most current hard disk drives spin at between 5,400 and 10,000 RPM.

Modern hard drives can transfer 80 megabytes of data per second.


The triangular-shaped head arm holds the read/write heads and is able to move the heads from the hub to the edge of the drive.

There is one hard arm per read/write head and all of them are lined up and mounted to the acuator as a single unit.


The head arm is controlled by an actuator - which has to be incredibly accurate. 30,000 tracks can be stored within one inch of space on a platter.

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 via its interface to the computer.


Hard drives are typically air-sealed to enable the heads to float and to avoid contamination from dust.

The air inside the the hard drive enclosure is in constant motion and passes through the filter to remove any leftover contaminants from the manufacturing process and any particles or chemicals that may have somehow entered the enclosure.

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