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Just open up the computer, and plug the drive in (while the computer's off, :D ). As long as you don't switch the drives around, it shouldn't recognize the OS on the other HDD. Even if it did, it probably wouldn't work if you tried to boot into it (different drivers and such on the other HDD). But you should check for the speed (RPM) of the drive first (it should be in a manual and\or on the drive itself), as it will slow down your other drive if it's slower and on the same channel (which it most likely will be).