The 3.25-inch disk was a floppy disk introduced in 1983 for use in the Tabor TC 500 Drivette disk drive. It was a double-sided disk, with an unformatted capacity of 500 KB.
It was similar in design to the 5.25-inch minifloppy disk, and in fact the drive could replace the 5.25-inch disk drive in a PC using the same cable. In addition to the Tabor Drivette, there was a prototype 3.25-inch disk drive system for the Coleco Adam computer (that at that time was using High Speed Digital Data Pack tape drives) and a 3.25-inch drive was used in the rare Seequa Chameleon 325 computer.
Producers of the disks included Tabor, Dysan, and 3M. However, Dysan seems to have been the main producer of disks, which it labelled as the ‘Flex Diskette’, since like the 5.25-inch disk, the disk’s envelope was flexible.
The 3.25-inch disk came about at a time when there were a number of competing designs of microfloppy disk, such as the 3.5-inch microfloppy and the 3-inch Compact Floppy, and it doesn’t seem to have lasted very long in the marketplace.