What expansion slot?
I think he means the side expansion slot on the A500. :-)
as Matt said though, you can add an SCSI interface to the A570 or use a harddrive controller with an external SCSI CD-Rom. (The down side of the SCSI CD-Rom is that you wont have a CDTV compatiable unit.
Other than that most none Commodore SCSI controllers did have a pass-through, so just dont use an A590.
