Well, I got a 2.1 gb working in my A600 with 37.350 KS 2.0 Roms. It wouldn't boot from it straight away, because it was too slow to boot straight up, but after a warm reset it would work. You have to be very quick with the warm reset though, otherwise the hard drive will spin down and the A600 won't detect it. Just tap the keys so it resets but the drive doesn't spin down and see whether that works.
You could also try getting KS 3.1 roms, they have a built in wait for hard drives, so it should work (assuming it's functional). :-)
