If you purchase one of the recently introduced iMacs with a Fusion Drive, and you boot that iMac into Target Mode, it will only mount as a drive when connected to another Mac running OS X 10.8.2 or later. This is a major change to the way Target Mode has worked in the past, where once a Mac was booted into Target Mode, it didn’t matter what version of OS X was running on the Mac you connected it to.