Thanks. We may be talking about slightly different things, however. I was wondering about the compatibility of the drive itself rather than the media. I.e. can you connect a M0131 external 800K drive to a stock Mac 128K or unenhanced 512K, and expect it to work, as long as you only read/write 400K MFS disks?

Thinking about it further, the answer must be yes, otherwise there would be no way to load that HD20 init.

That's good news, as it means my floppy emulator can identify itself as a double-sided drive in all cases, and the 128K/512K will just ignore the second side. Of course it will only work if you select a 400K MFS disk image.