Isn't there something different about the new 21.5" & 27" iMacs in that the temp sensor is removed and they now use the drive's sensor so only certain drive upgrades work because w/o the correct pins active in the drive the iMac fans run all the time?
Annoying Apple crap once again. The little external sensor worked with any drive, now we have to worry about buying the correct upgrade drive. Just one more way Apple tries to discourage user upgrades. 🙁
Cheers,
EDIT: ^^ yep I just saw he confirmed it.