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What is Hard Disk?

A hard disk drive (often shortened as hard disk, hard drive, or HDD) is a non-volatile storage device that stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media..

Hard Disk Basics:

Hard disks are organized as a concentric stack of platters. The data is stored on concentric circles on the surfaces known as tracks. Sections within each track are called sectors. A sector is the smallest physical storage unit on a disk and typically it will hold 512 bytes of data. The disk itself can't handle smaller amounts of data than one sector.

Figure 1: Hard disk schematic

Electromagnetic read/write heads are positioned above and below each platter. As the platters spin, the drive heads move in toward the center surface and out toward the edge. In this way, the drive heads can reach the entire surface of each platter. Reading from 2 tracks implies a realignment of the reading heads, thus it takes longer than reading a single track.

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