MCD-1 Micro Cassette
The MCD-1 Micro Cassette was one of several odd-sized floppy formats that never caught on. It was 3 inches in size, and different from the CF-2 Compact Floppy Disk (and predated it). It was invented in Hungary in 1973, but took until 1979 to complete development of a working prototype. It was considered for some computers such as the Commodore line, but ended up mostly being used within the eastern bloc. It never caught on very much.
MFM encoding was used for a 149.6 KB capacity, or FM encoding for a 74.8 KB capacity. There were 45 tracks with 13 sectors per track, with 256 bytes (in MFM encoding) or 128 bytes (in FM encoding) per sector.