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.
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