I thought of another issue last night. The main advantage of a HD20 Emu over SCSI2SD is that it would work on the Mac 512k and 512ke, right? But the Mac 512k lacks the HD20 Init in ROM, and the only way to load it is from a floppy, and that's probably going to require a real floppy in the internal drive, or two Floppy Emus. I don't think the hardware can act as an HD20 and a floppy at the same time, and if it can, it could only act as a chain with the emulated floppy at the end, and the ROM routines in the Mac 512k don't know how to talk to drives other than the first one in the chain.

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I don't have an HD-20 to test, but can a 64k ROM 512k boot from an external floppy drive hanging off the daisy-chain port of the HD-20? A 512k *can* boot from an external floppy otherwise. Perhaps the HD-20 on power-on comes up with the crossover circuit initialized so the floppy is the thing the Mac "sees" and only becomes UN-transparent after the HD-20 init fires off the the drive discovery routine in its modified version of the .sony driver. *If* that were the case then if you could make the FloppyEmu able to "multitask" being both an HD-20 and a floppy (You'd have to teach the hardware to recognize the phase line dance that switches the crossover circuitry but other than that it seems it should be doable... said by someone who didn't build it, of course. The two emulations don't really have to run at the same time, as long as you have enough memory to store state when the Mac switches drives it should be doable? The Smartport emulator I linked earlier actually emulates four separate Smartport HDs, each hardcoded to point to a 32MB partition on the CF drive, by watching the drive select signals.) you could boot a 512k diskless simply by having an HD-20 boot system image pointed to by the floppy emulation side of the house.