I recently swapped out a knackered CDRW drive from a Firewire case and replaced it with a new 160GB IDE hard drive. The drive wouldn't spin up properly (it was intermittant) and I believe that the Firewire enclosure wasn't providing enough power. However I put the hard drive into another external CDRW enclosure and it worked fine.

It will probably be a case of experimenting but certainly it can be done and if everything is IDE then there shouldn't be any interfacing issues.