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Hard disks are magnetic storage devices on which you can store computer data like your program files, OS files, Documents etc. At the simplest level, a hard disk is not that different from a cassette tape. Both hard disks and cassette tapes use the same magnetic recording techniques.

In a hard disk, a magnetic recording material is layered onto a high-precision aluminium or glass disk. The hard-disk platter is then polished to mirror-type smoothness, A single hard disk usually consists of several platters. Each platter requires two read/write heads, one for each side. All the read/write heads are attached to a single access arm so that they cannot move independently, the read/write head `flies' over the disk, never actually touching it.

There are two ways to measure the performance of a hard disk:

Data rate — The data rate is the number of bytes per second that the drive can deliver to the CPU. Rates between 5 and 40 megabytes per second are common.

Seek time — The seek time is the amount of time between when the CPU requests a file and when the first byte of the file is sent to the CPU. Times between 10 and 20 milliseconds are common.

The other important parameter is the capacity of the drive, which is the number of bytes it can hold like 20GB or 40GB.

Compiled by G. Rajah

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