Single Sided, Double Density
SSDD originally referred to Single Sided, Double Density, a format of (usually 5ΒΌ") floppy disk which could typically hold 35-40 tracks of nine 512-byte (or 18 256-byte) sectors each. Only one side of the disc was used, although some users did discover that punching additional holes into the disc jacket would allow the creation of a "flippy" disc which could be manually turned over to store additional data on the reverse side.
Single-sided disks began to become "obsolete" after the introduction of IBM PC DOS 1.1 in 1982, which added support for double-side diskette drives with a boosting capacity of 320 KB to the IBM 5150 PC, which had been released the year before, but originally only with single-sided 160 KB drives. In 1983 PC DOS 2.0 pushed the formatting capacity to 180 KB respectively 360 KB by utilizing 9 instead of only 8 sectors per track. Ironically this same year, Commodore released a floppy disk system that could store 1 MB of data, but it was not well received in part because their users felt that it was overkill.
Read more about this topic: Floppy Disk Format
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