The GCC HyperDrive mechanism cannot possibly be an ST-506. Why? Because the ST-506 was a 5MB drive and GCC only release 10MB and 20MB variants of the HyperDrive. Also, the ST-506 and ST-xx series drives were made by Seagate. But the label on my 20MB GCC Hyperdrive indicates MMi was the manufacturer:




Nevertheless, I cannot find much of anything on MMi. Hence, I am curious if MMi didn't just slap a label on an NEC drive. If you have a look at my Flickr photo of the Hyperdrive, it looks very similar (yet a tad different) to the NEC D5126H shown half way down this web page:

http://www.redhill.net.au/d/d-a.html