The 8-inch disk
In 1967, IBM gave their San Jose, California storage development center a task to develop a reliable and inexpensive system for loading microcode into their System/370 mainframes in a process called Initial Control Program Load (ICPL). The System/370 was IBM's first computer system family to make extensive usage of volatile read/write semiconductor memory for microcode, so for most models, whenever the power was turned on the microcode had to be loaded (System/370's predecessor, System/360, generally used non-volatile read-only memory for microcode). IBM also wanted inexpensive media that could be sent out to customers with software updates.
IBM Direct Access Storage Product Manager Alan Shugart assigned the job to David L. Noble, who tried to develop a new-style tape for the purpose, but without success. Noble's team developed a read-only, 8-inch-diameter (200 mm) flexible diskette they called the "memory disk", holding 80 kilobytes of data. The original disk was bare, but dirt became a serious problem so they enclosed it in a plastic envelope lined with fabric that would remove dust particles. IBM introduced the diskette commercially in 1971.
The new device, developed under the code name Minnow and shipped as the 23FD, was a standard part of System 370 processing units (IBM internally used another device, code named Mackerel, to write boot disks for distribution to the field). It also was used as a program load device for other IBM products such as the 2835 Storage Control Unit.
Alan Shugart left IBM and moved to Memorex where his team in 1972 shipped Memorex 650, the first read-write floppy disk drive. The 650 had a data capacity of 175 kB, with 50 tracks, 8 sectors per track, and 448 bytes per sector. The Memorex disk was "hard-sectored", that is, it contained 8 sector holes (plus one index hole) at the outer diameter (outside data track 00) to synchronize the beginning of each data sector and the beginning of a track.
In 1973 IBM shipped its first read/write floppy disk drive, the 33FD, as a part of the 3740 Data Entry System, code named "IGAR", designed to directly replace IBM's punched card ("keypunch") data entry machines. A significant feature of IBM's read/write disk media was the use of a teflon-lubricated fabric liner to lengthen media life. In 1976 media supplier Verbatim enhanced resilience further by adding a teflon coating to the magnetic disk itself. The new system used a different recording format that stored up to 250¼ kB on the same disks. Drives supporting this format were offered by a number of manufacturers and soon became common for moving smaller amounts of data. This disk format became known as the Single Sided Single Density or SSSD format. It was designed to hold just as much data as one box of 2000 punch cards. The disk was divided into 77 tracks of 26 sectors, each holding 128 bytes. Note that 77 × 26 = 2002 sectors.
Also in 1973, Shugart founded Shugart Associates which went on to become the dominant manufacturer of 8-inch floppy disk drives. Its SA800 became the industry standard for form factor and interface.
In 1976 IBM introduced the 500 KB Double Sided Single Density (DSSD) format, and in 1977 IBM introduced the 1-1.2 MB Double Sided Double Density (DSDD) Format.
Other 8-inch floppy disk formats such as a Burroughs 1 MB unit failed to achieve any market presence.