The proper name for this interface is actually ST-506/ST-412.
This is a very old interface, used to connect storage devices to a motherboard.
This interface predates IDE, and is not used anymore for harddrives. It was developed in the 1980's by Seagate, a storage technology company.
Typical harddrive sizes are 5MB, 10MB and 20MB.
MFM actually refers to the encoding method used for storing data on the disk. MFM is an acronym for "Modified Frequency Modulation".
Another encoding method used with this interface is RLL. (RLL encoding is possibly still used on modern IDE drives)
ST-506/ST-412 cables look like skinny IDE cables. (About half the width?) A single ST-506/ST-412 harddrive needs two cables, one is 20 pins, the other is 34 pins.
The biggest problem with this interface1? is that there is no logic board mounted on the harddrive. The harddrive is controlled
by a separate controller card. Each harddrive requires its own unique controller card. Don't expect the controller cards to have any markings on them to indicate which harddrives they belong to/can work with. You have an ST-506/ST-412 harddrive but no controller card? Bad luck, use it as a boatanchor or a doorstop
Apparently floppy drives still use this protocol (with MFM encoding)
ST-506/ST412 hardrives are BIG, NOISY, HEAVY and UNRELIABLE2?. (About the size of a CD-ROM drive. They sound like typewriters IIRC :)
"The last time I fiddled with an MFM harddrive was about 15 years ago, so most of this is from memory and whatever I could quickly google up ;^)" -- MarcelVanDeSteeg
1? Besides the slow speed and the small storage capacity of the harddrives :D
2? I ran into a working example the other day, buried in the internals of an old 4Mhz 8088 XT.