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(computer science) computer hardware that holds and spins a magnetic or optical disk and reads and writes information on it

computer hardware that holds and spins a magnetic or optical disk and reads and writes information on it

(Or "hard disk drive", "hard drive","floppy disk drive", "floppy drive") A peripheral devicethat reads and writes hard disks or floppy disks. Thedrive contains a motor to rotate the disk at a constant rateand one or more read/write heads which are positioned over thedesired track by a servo mechanism. It also contains theelectronics to amplify the signals from the heads to normaldigital logic levels and vice versa.In order for a disk drive to start to read or write a givenlocation a read/write head must be positioned radially overthe right track and rotationally over the start of the rightsector.Radial motion is known as "seeking" and it is this whichcauses most of the intermittent noise heard during diskactivity. There is usually one head for each disk surface andall heads move together. The set of locations which areaccessible with the heads in a given radial position are knownas a "cylinder". The "seek time" is the time taken toseek to a different cylinder.The disk is constantly rotating (except for some floppy diskdrives where the motor is switched off between accesses toreduce wear and power consumption) so positioning the headsover the right sector is simply a matter of waiting until itarrives under the head. With a single set of heads this"rotational latency" will be on average half a revolutionbut some big drives have multiple sets of heads spaced atequal angles around the disk.If seeking and rotation are independent, access time is seektime + rotational latency. When accessing multiple trackssequentially, data is sometimes arranged so that by the timethe seek from one track to the next has finished, the disk hasrotated just enough to begin accessing the next track.See also sector interleave.Early disk drives had a capacity of a few megabytes and werehoused inside a separate cabinet the size of a washingmachine. Over a few decades they shrunk to fit a terabyteor more in a box the size of a paperback book.The disks may be removable disks; floppy disks always are,removable hard disks were common on mainframes andminicomputers but less so on microcomputers until the mid1990s (?) with products like the Zip Drive.A CD-ROM drive is not usually referred to as a disk drive.Two common interfaces for disk drives (and other devices) areSCSI and IDE. ST-506 used to be common inmicrocomputers (in the 1980s?).