To attempt to straighten all this out:
The "ST-506" interface is actually a superset of the old Shugart floppy bus (which PCs still use, incidentally). The wide cable has the control signals, and the narrow 20-pin cable carries the data (raw data with MFM or RLL applied and all the formatting marks and ECC bytes present).
Just like a floppy drive, to read and write, you set a direction, send pulses to the drive to move the head stepper motor, then pick a head, set the write gate and start reading or writing. The "WD1010" on the GCC board is what manages all this; all the Mac has to do is tell it what cylinder/head/sector to go to, and it goes. (You can also do tricks with the ST-506 interface like "buffered seek", where you can send pulses to the drive as fast as it can take them, then tell it to seek; if you ever see a drive supporting the "ST-412" interface, that basically means the drive supports buffered seek. The original ST-506 couldn't do this because it didn't have a CPU!)
Incidentally, the ESDI interface some high-end drives of the era used is similar to ST-506/412, but supports a lot more features. It's to the older spec what SmartPort is to Disk ][. It's not likely you'd ever see an ESDI drive in a Mac, though some makers (CDC and Maxtor in particular) slapped SCSI interfaces on their ESDI drives once it was obvious ESDI was dying.
The hard disk in JDW's unit isn't a MiniScribe. If it were, it'd be something of the 8425 persuasion, which (among other things) has the ICs facing out on the logic board, and has a sticker on top with the date of manufacture and "MiniScribe - QUALITY" printed on it. MMI doesn't seem to be well-known, but I did find some info in them in the old TheRef guide, as "Micro Memories", and it would seem they were based in Chatsworth, CA -- also home to Pertec, Micropolis and CMI (all related in one way or another). MMI in particular was either a subsidiary or spinoff of CMI, since the drive looks like a smaller CMI 6000 (the drive that caused IBM a huge amount of trouble back in 1985). The golden top, the cheap serial number tag and the red and white warranty seal are the giveaways.
The "ST-506" interface is actually a superset of the old Shugart floppy bus (which PCs still use, incidentally). The wide cable has the control signals, and the narrow 20-pin cable carries the data (raw data with MFM or RLL applied and all the formatting marks and ECC bytes present).
Just like a floppy drive, to read and write, you set a direction, send pulses to the drive to move the head stepper motor, then pick a head, set the write gate and start reading or writing. The "WD1010" on the GCC board is what manages all this; all the Mac has to do is tell it what cylinder/head/sector to go to, and it goes. (You can also do tricks with the ST-506 interface like "buffered seek", where you can send pulses to the drive as fast as it can take them, then tell it to seek; if you ever see a drive supporting the "ST-412" interface, that basically means the drive supports buffered seek. The original ST-506 couldn't do this because it didn't have a CPU!)
Incidentally, the ESDI interface some high-end drives of the era used is similar to ST-506/412, but supports a lot more features. It's to the older spec what SmartPort is to Disk ][. It's not likely you'd ever see an ESDI drive in a Mac, though some makers (CDC and Maxtor in particular) slapped SCSI interfaces on their ESDI drives once it was obvious ESDI was dying.
The hard disk in JDW's unit isn't a MiniScribe. If it were, it'd be something of the 8425 persuasion, which (among other things) has the ICs facing out on the logic board, and has a sticker on top with the date of manufacture and "MiniScribe - QUALITY" printed on it. MMI doesn't seem to be well-known, but I did find some info in them in the old TheRef guide, as "Micro Memories", and it would seem they were based in Chatsworth, CA -- also home to Pertec, Micropolis and CMI (all related in one way or another). MMI in particular was either a subsidiary or spinoff of CMI, since the drive looks like a smaller CMI 6000 (the drive that caused IBM a huge amount of trouble back in 1985). The golden top, the cheap serial number tag and the red and white warranty seal are the giveaways.