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Disk density is a capacity designation on magnetic storage, usually floppy disks. Each designation describes a set of characteristics that can affect the areal density of a disk or the efficiency of the encoded data.
Hard disk drives store data in the magnetic polarization of small patches of the surface coating on a disk. The maximum areal density is defined by the size of the magnetic particles in the surface, as well as the size of the "head" used to read and write the data. In 1956 the first hard drive, the IBM 350, had an areal density of 2,000 bit/in 2.
8-inch floppy disk, inserted in drive, (3½-inch floppy diskette, in front, shown for scale) 3½-inch, high-density floppy diskettes with adhesive labels affixed The first commercial floppy disks, developed in the late 1960s, were 8 inches (203.2 mm) in diameter; [4] [5] they became commercially available in 1971 as a component of IBM products and both drives and disks were then sold ...
Areal density (the number of bits that can be stored in a certain area of the disk) has been increased over time by increasing both the number of tracks across the disk, and the number of sectors per track. The latter will increase the data transfer rate for a given RPM speed.
If the air density is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 3,000 m (9,800 ft). [ 136 ]
The term density has a double meaning for floppy disks. Originally, single density and double density indicated a difference in logical encoding on the same type of physical media: FM for single, and MFM for double. Subsequent use of the term "density" referred to physical characteristics of the media, with MFM assumed to be the logical format ...
Disks varied sectors / track and disk speed to keep consistent bit density across tracks [26] Double 2 512 1,196 kB [27] Tandy TRS-80 5 1 ⁄ 4 inch Single 1 35 10 256 soft 88 kB 300 FM Model 1/3/4 5 1 ⁄ 4 inch Double 1 40 18 256 180 kB MFM Model 1/3/4P 5 1 ⁄ 4 inch Double 2 40 18 256 360 kB MFM Model 4D 8 inch Double 1 77 26 256 500 kB MFM ...
The first commercial digital disk storage device was the IBM 350 which shipped in 1956 as a part of the IBM 305 RAMAC computing system. The random-access, low-density storage of disks was developed to complement the already used sequential-access, high-density storage provided by tape drives using magnetic tape.