This was a 14-inch fixed magnetic disk unit Toshiba started shipping in June 1977. Average access time was 90msec, and weight was 30Kg. Storage capacity was 12MB for the MK-100F and 36MB for the MK-300F.
Toshiba's previous magnetic disk units were the disk pack type, with replaceable recording media, but the MK-100F and MK-300F were the fixed magnetic disk type in which the recording media could not be replaced. Switching to the fixed type made greater unit miniaturization possible, eliminated the need for preventive maintenance, and improved reliability, and thus this unit was built into the office computers of the time. It could be mounted in a standard 19-inch rack, and began to be used even in minicomputers. It was also built into the JW-10, which was developed by Toshiba as Japan's first Japanese language word processor. This unit was the foundation for Toshiba's subsequent magnetic disk units.