Gary, it depends on which model you have. The later QS/MDD models have a spare 5.25" external bay - the earlier ones appear to have a 3.5" external bay (for a zip). At least that's what I could determine from everymac.com. Not sure what the internal bits look like, but you should have a look. From one set of installation instructions which I can no longer find, the picture seemed to show a smaller drive tray/housing in the earlier model. Whether it's possible to modify the innards to accommodate two 5.25" drives, I don't know. All the references for two optical drive installations on XLR8YourMac.com are to the MDD's.