<P> The first disk drive, the 1957 IBM 350 disk storage had ten 100 character sectors per track; each character was six bits and included a parity bit . The number of sectors per track was identical on all recording surfaces . There was no recorded idenfifier field (ID) associated with each sector . </P> <P> The 1961 IBM 1301 disk storage introduced variable length sectors, termed records by IBM, and added to each record a record address field separate from the data in a record (sector). All modern disk drives have sector address fields, called ID fields, separate from the data in a sector . </P> <P> Also in 1961 Bryant with its 4000 series introduced the concept of zoned recording which allowed the number of sectors per track to vary as a function of the track's diameter - there are more sectors on an outer track than on an inner track . This became industry practice in the 1990s and is standard today . </P> <P> The disk drives announced with the IBM System / 360 in 1964 detected errors in all fields of their sectors (records) with a cyclic redundancy check (CRC) replacing parity per character detection of prior generations . IBM's sectors (records) at this time added a third field to the physical sector, a key field to aid in searching for data . These IBM physical sectors, called records, have three basic parts, a Count field which acts as an ID field, a Key field not present in most disk drive sectors and a Data field, frequently called the CKD format for a record . </P>

What is a sector on a hard drive