You can use both the Apple 3.5 drive usually found on the IIgs with the external 800k 3.5" drives for the Macs on both the IIgs and Macs. However, if you use the Mac drive on a IIgs, you don't have an eject button and have to eject from the IIgs Finder--or use a paperclip. Thes drives cannot be used on the IIc--only the unidisk was designed for that. The unidisk can be used on a IIgs, though.