You are talking about an external HD in its own Freecom case? If so, may I ask ..... why?
If you want to swop HDs you might consider another unit and leave the old one as-is.
Are you sure there are no minute X-point screws at the ends? or hiding behind rubber feet? If all you have to play with are the slots then see if a small screwdriver will move a hook in a slot. It doesn't have to move far. If it will move then make enough wooden wedges to "open" more than one catch at a time. Early Epson printers used to open this way.