This is strange. I have a SATA drive that works when it's in an external USB enclosure but when I put the same SATA drive in one of the bays in my Mac Pro, the Mac Pro won't start up. Specifics: -I've reformatted the drive (with GUID and with Mac Partition Map), with no change in behavior -A different drive in the same Mac Pro drive bay works fine, so it's not the drive bay -the suspect drive in any of drive bay keeps the computer from booting (grey screen, no apple) -the suspect drive works fine (and tests fine with disk tools and Drive Genius) when it's in the external USB enclosure WTF??!!?!?! What would make the drive work fine in an external enclosure yet prevent the Mac from starting up when it's put into a drive bay??!?!?!