Yup, for sure you'd have fundamental problems if you wanted to use 400K or 800K disks, due to the variable rotation speed used with those disks. For 1440K disks I think it should be possible to interface a PC floppy drive, at least in theory, but it would be pretty complicated. Like you said, it's a radically different interface. You'd need some kind of microcontroller/CPLD/FPGA/something that handled all the communication with the Mac, and simulated those internal floppy disk registers. Then it would need to translate to equivalent but completely different communication with the PC floppy drive. For some stuff like sensing track 0, this would be relatively straightforward. Other stuff would be harder, because there's no PC floppy equivalent. For example, what should you do when the Mac tries to eject the floppy? Or when the Mac tries to check the track stepping progress state, which has no equivalent on a PC floppy drive? But if you're determined enough, I think those issues could be overcome. It would be an interesting project. Good luck.