Several of those "drive"s were single board
CP/M computers, including a
few that were Ampro. The CP/M computer could read and write MFM
diskettes, and communicate with the Mac. Most used SCSI or serial for the
communication. But they were marketed as MFM "DRIVE"s.
I believe that the Indus GT drives for the Atari and
Commodore 8bits fall into this catagory.
The Atari Indus GT was undoubtedly MFM, but the Commodore Indus is definitely
GCR (I own one).