Carefull, some A600s came with 2.04 ROMs (no HD, no PCMCIA9, so one should 1st check wether atleast 2.05 is installed.
3.5-HD can be made fit, just leave out the craddle and buy an adaptor-cable (Vesalia).
If the A600 is a non-HD version you probraly don't have the 2.5-craddle nor the the small ribbon-cable and have to buy these (atleast the cable) before installing an HD.