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Hard Disk Drive is the mechanism that controls the positioning,
reading, and writing of the hard disk, which furnishes the largest
amount of data storage for the PC. Although the hard disk drive
and the hard disk are not the same thing, they are packaged as a
unit and so either term is sometimes used to refer to the whole
unit.
There are several interface standards for passing data between
a hard disk and a computer. The most common are IDE and SCSI.
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A single hard disk/drive
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.
Each platter has the same number of tracks, and a track location
that cuts across all platters is called a cylinder. For example,
a typical 84 megabyte hard disk for a PC might have two platters
(four sides) and 1,053 cylinders
A hard disk/drive unit comes with a set rotation speed varying
from 4500 to 7200 rpm. Disk access time is measured in milliseconds.
Although the physical location can be identified with cylinder,
track, and sector locations, these are actually mapped to a logical
block address (LBA) that works with the larger address range on
today's hard disks.
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