It's been a while so my memory may be fuzzy, but I got my first Mac at work in Nov '83, and bought my own Mac in May '84. A bit later Apple came out with the first 3.5" Apple][ drive which is 800K with a built-in 6502 of its own. Removing the Apple][ controller card left a drive which could be plugged into a Mac, and which the Mac (64K ROM) recognized as a double-sided disk and formatted MFS 800K. There really wasn't anything more to it than that.
Later 1.4 MB and possibly other model 800K drives needed the 128K Mac Plus ROMs to work properly. My old 128 has Mac Plus ROMs in it now, so I can't easily go back to verify the gory details. That poor Mac was the guinea pig for a Dr. Dobbs style 512K upgrade, development of the Beck-Tech 1MB RAM upgrade, the SCSI interface and chassis design of the Micah internal hard drive, and many other hacks. It still runs, by the way. A tribute to Burrell Smith's solid engineering.
Later 1.4 MB and possibly other model 800K drives needed the 128K Mac Plus ROMs to work properly. My old 128 has Mac Plus ROMs in it now, so I can't easily go back to verify the gory details. That poor Mac was the guinea pig for a Dr. Dobbs style 512K upgrade, development of the Beck-Tech 1MB RAM upgrade, the SCSI interface and chassis design of the Micah internal hard drive, and many other hacks. It still runs, by the way. A tribute to Burrell Smith's solid engineering.