Quad density
Quad Density , more rarely in full Quadruple Density , abbreviated QD or 4D , is one of several possible recording densities on floppy disks . It is only used for 5.25 "floppy disks with a physical storage capacity of 1,000 KB .
With normal formatting with 80 tracks and 9 sectors per track, 5.25 "QD disks have a capacity of 720 KB with a track density of 96 tpi . QD disks are usually backwards compatible, so they can also be formatted as DD disks if required QD or HD-capable floppy disk drives are required for writing to and reading from QD floppy disks, but these have long been the state of the art. QD floppy disks are rarely found. They could not establish themselves on the market and were replaced by the emerging 3 , 5 "-DD disks and replaced in the 5.25" area by the 5.25 "-HD disks.
The MS-DOS and Windows 95/98 / 98SE operating systems cannot inherently format QD disks and recognize an unformatted QD disk as a DD disk. However, floppy disks formatted with FAT12 can be written to and read from. Linux handles QD floppy disks like any other floppy disk; formatting is possible with the fdformat command .