I too have confirmed that a stock 128K will not work with the HD20 INIT. Pina says that the 128Ke (128K with 128K ROMs) works just fine. JDW is correct that it may in fact work since the 128K does not have to load the HFS & HD20 drivers into RAM. However, I posed this question to Phil Beesley over at http://www.VintageMacWorld.com (arguably the current expert on the HD20) and he suggested I think rightly so: "In theory, you could use a HD20 with a Mac 128; in practice, the experience will be horrible because the Finder will try to cache all of the directory information and use up all of the RAM. Even when using floppy disks with a small number of folders/files, I'd expect nasty crashes when the Finder or an application runs out of RAM."
As for formatting an 800K MFS disk on a 64K ROM Mac, it is definitely possible. JDW, I saw those instructions somewhere too, but it was after I discovered it on my own ... where was that link? In any event it was second nature to my 128K and 512K so I never thought to document the process. I thought it was just a matter of attaching a proper drive as Larry Pina first posits and it just worked. In fact that's how it works for me. I simply attached an A9M0106 external 800K drive to my stock 512K Mac. Then I started up with a 400K MFS disk. Then I inserted a blank 800K disk and was asked to initialize it. Once It had been initialized, I had an 800K MFS disk, confirmed by the lack of an extra pixel under an HFS system formated startup disk. I NEVER went through the extra steps of first formatting the disk with a system and finder. So I've done it and used it ... actually it was invaluable in using my MacTerminal file transfer tutorial. However, I will be the first to admit that I did not note the exact steps I took to accomplish it since I assumed there were no special steps other than I've stated. Rather than speculate and add to the confusion, as soon as I have time to set up my Macs, I will make this a priority to determine since you were unable to reproduce it with your 64K ROM.
As for formatting an 800K MFS disk on a 64K ROM Mac, it is definitely possible. JDW, I saw those instructions somewhere too, but it was after I discovered it on my own ... where was that link? In any event it was second nature to my 128K and 512K so I never thought to document the process. I thought it was just a matter of attaching a proper drive as Larry Pina first posits and it just worked. In fact that's how it works for me. I simply attached an A9M0106 external 800K drive to my stock 512K Mac. Then I started up with a 400K MFS disk. Then I inserted a blank 800K disk and was asked to initialize it. Once It had been initialized, I had an 800K MFS disk, confirmed by the lack of an extra pixel under an HFS system formated startup disk. I NEVER went through the extra steps of first formatting the disk with a system and finder. So I've done it and used it ... actually it was invaluable in using my MacTerminal file transfer tutorial. However, I will be the first to admit that I did not note the exact steps I took to accomplish it since I assumed there were no special steps other than I've stated. Rather than speculate and add to the confusion, as soon as I have time to set up my Macs, I will make this a priority to determine since you were unable to reproduce it with your 64K ROM.