Just like moving games from one computer to another requires a re-installation, so does moving games from one hard drive to another. You can hand-hack it in the registry but that will take you hours for each game. Or you can copy your saved game files to the external drive, un-install the game, re-install to the directory you want it on, and then copy your saved game back over to the file it came from, in about thirty minutes per game.
Edit: If your hard drive is also containing videos and movies, it's a heck of a lot easier to move those over to a secondary drive.
Edited, May 11th 2012 2:02pm by catwho