Yep! That's an MMI drive. You are lucky you got one that was made just right. A lot of them had trouble with excess lubricant on the disk surface spinning to the outside edge where the heads would get stuck in it. Back then it seemed like all disk drive mfrs would go through periods where they couldn't build a drive that would run for more than 6 weeks. A working Hyperdrive is a real rarity. In the day, they marked the serious (and wealthy) Mac user. The thread about how GC got control of the system to patch disk I/O at boot time is correct. Hyperdrives have PC/AT MFM controller chips interfaced to the 68000 data & address buss on the clip-on card. No DMA, if I recall, but still way faster than a floppy or an HD20.

As time passes, it gets harder to find running examples of old drives. I thought I'd pick up a spare SCSI laptop drive last time I was at Goodwill (Austin's local computer recycler), but they said they hadn't seen one for a couple of years. You have to grab spares while things are available. Right now, it's G4s that are being recycled. G3s are getting scarce, though they were everywhere last year. If you need a G3 for parts, grab one while they're available and cheap. 68Ks are seldom seen now, and collectors are starting to bid the price up.