Hard Disk Drives
Hard disk drives are mass storage devices and all modern computers have at least one hard disk drive. The first hard disk drives were small in capacity, physically large, and expensive in comparison with the hard disk drives of today. They were about 4 inches tall, 5.25 inches wide, and 8 inches long. The first personal computer to include a hard disk drive was IBM's XT computer of 1981. This computer came with a 10-MB hard disk drive installed in it; but the first hard disk drives were released in the late 1970s. These drives were initially called fixed disks because they were not removable.
Hard
disk drives are composed of several platters, matched to a collection
of R/W heads and an actuator. Unlike floppy disk drives, a hard disk
drive is sealed, although it has an aperture with an air filter. As
it is sealed, it cannot be contaminated by dust or smoke from the
surrounding environment while the aperture allows the air pressure
to be equalized between the interior and the exterior of the drive.
The aim of the hard disk drive is to access data stored on a flat surface quickly and directly. To do this, two different motions are required. As the disk spins, the R/W heads move across the platter perpendicular to the motion of the disk. The R/W heads are mounted on the ends of the actuator arms. A critical element in hard disk drive design is the speed and accuracy of these actuator arms.