The floppy drive mechanisms in both cases are pretty dumb devices. PC floppy drives are actually dumber than Mac floppy drives -- about the only thing they can do by themselves is reset the heads to track 0 on power-up, and the bus they use is pretty much the same as the old Shugart SA400 from 1976 -- one step at a time, no direct control of phases. Apple 3.5" floppy drives have registers inside them for various things, I've forgotten what, so they're marginally smarter than PC drives. (I do know they use the stepper motor phases as a register select bus.)
The Disk II was the dumbest of all, having nothing more than the stepper motor and a head amplifier, and I think that's what Anonymous Freak had in mind.
Anyway, the real issue here is what the controller does with the data once it receives it. The IWM, just like the Disk II before it, is just a state machine that pulls raw bits off the disk and expects the OS to make sense of them. This makes it quite flexible; you could probably read an MFM disk on an IWM if you knew what bits to tweak. (I'm not sure what magic the SWIM chip uses for MFM, but I would imagine the reason it wasn't done sooner is that either the IWM or the Mac itself couldn't handle 500-kbit MFM data reliably.)
PCs, however, use a chip that traces its ancestry back to the NEC uPD765 that was in the original IBM PC/XT; specifically, they're variants of either the Intel 82072 or Western Digital 37C65 PC/AT floppy controllers, depending on whose Super I/O chip you have. The 765 is easy to program and supports DMA, but it's also incredibly inflexible compared to the IWM; it only supports FM or MFM in the standard IBM "midrange" formats, and all data appears "cooked" -- raw disk access is pretty much impossible without hacks, as is reading a disk that strays too far out of the IBM specs. It also expects to use the same data rate across the entire disk (CAV mode); Mac 800k disks are zoned CLV, making them even trickier to read.
Pretty much all USB floppy drives use this particular setup, as it's well-known and easy to implement using off-the-shelf parts, so reading anything "weird" on them is out of the question. There's also the odd LS120/240 drive floating around still; they're very fast in floppy mode, but also limited to MFM (though they don't use 765s as far as I know).
Back in the day, there was a piece of hardware, the Copy II PC Deluxe Option Board, that got around all this by cutting the 765 out of the loop and using its own custom ICs to process the raw bits coming off the disk. It could read practically anything if it knew what to look for. Unfortunately, they're kind of hard to find now, and would require an older machine with ISA to be useful. That said, I have one, and it does actually work once you find the utilities...
Failing that, your best bet is to use an older Mac as a go-between. One of the reasons I still have my 8600 is that it can read and write 800k disks for my ROM 0 IIgs. (That and my 475's floppy controller is messed up.)
The Disk II was the dumbest of all, having nothing more than the stepper motor and a head amplifier, and I think that's what Anonymous Freak had in mind.
Anyway, the real issue here is what the controller does with the data once it receives it. The IWM, just like the Disk II before it, is just a state machine that pulls raw bits off the disk and expects the OS to make sense of them. This makes it quite flexible; you could probably read an MFM disk on an IWM if you knew what bits to tweak. (I'm not sure what magic the SWIM chip uses for MFM, but I would imagine the reason it wasn't done sooner is that either the IWM or the Mac itself couldn't handle 500-kbit MFM data reliably.)
PCs, however, use a chip that traces its ancestry back to the NEC uPD765 that was in the original IBM PC/XT; specifically, they're variants of either the Intel 82072 or Western Digital 37C65 PC/AT floppy controllers, depending on whose Super I/O chip you have. The 765 is easy to program and supports DMA, but it's also incredibly inflexible compared to the IWM; it only supports FM or MFM in the standard IBM "midrange" formats, and all data appears "cooked" -- raw disk access is pretty much impossible without hacks, as is reading a disk that strays too far out of the IBM specs. It also expects to use the same data rate across the entire disk (CAV mode); Mac 800k disks are zoned CLV, making them even trickier to read.
Pretty much all USB floppy drives use this particular setup, as it's well-known and easy to implement using off-the-shelf parts, so reading anything "weird" on them is out of the question. There's also the odd LS120/240 drive floating around still; they're very fast in floppy mode, but also limited to MFM (though they don't use 765s as far as I know).
Back in the day, there was a piece of hardware, the Copy II PC Deluxe Option Board, that got around all this by cutting the 765 out of the loop and using its own custom ICs to process the raw bits coming off the disk. It could read practically anything if it knew what to look for. Unfortunately, they're kind of hard to find now, and would require an older machine with ISA to be useful. That said, I have one, and it does actually work once you find the utilities...
Failing that, your best bet is to use an older Mac as a go-between. One of the reasons I still have my 8600 is that it can read and write 800k disks for my ROM 0 IIgs. (That and my 475's floppy controller is messed up.)