Hard Disk Drive

Hard Disk Drive Musaad alangari-Turkey Alharbi-Khaled Almajnouni-Yousef Alanizy

A hard disk is part of a unit -- often called a disk drive, hard drive or hard disk drive -- that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. Today's computers typically come with a hard disk that can contain anywhere from billions to trillions of bytes of storage. A hard disk is actually a set of stacked disks, like phonograph records. Each disk has data recorded electromagnetically in concentric circles, or tracks, on the disk. A head, similar to a phonograph arm but in a relatively fixed position, writes or reads the information on the tracks. Two heads, one on each side of a disk, read or write the data as the disk spins. Each read or write operation requires that data be located, an operation called a seek. Data already in a disk cache, however, will be located more quickly. A hard disk/drive unit comes with a set rotation speed varying from 4,200 revolutions per minute to 15,000 rpm. Most laptop and desktop PCs use hard disks that fall between 5,400 rpm and 7,200 rpm, while hard disks at higher rpm can be found in high-end workstations and enterprise servers. Disk access time is measured in milliseconds. Although the physical location of data 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 hard disks.

back to the main page