AFAIK the PPC640 never hard a hard disk. There were a pair of D conenctors (DB25 and DC37 I think, I forget the gender) that carried the system bus. There is rumoured to be an expansion chassis for it, I've never seen one. I also beliecve that US versions diddn't have those connecotrs fitted, due to the FCC emissions regulations. I guess it would support an XT hard disk controller (with its own boot/BIOS ROM) connected there, but I've never seen it done.