Are you telling me your Xbox won't load an Xbox game? Error 21 is almost always a dash file missing. The xbox is set to boot from the disc drive before the hard drive, and therefore, it should be able to boot the Xbox game before even looking at the hard drive. What I'm saying is, connect the xbox hd into the xbox. Then, boot with a game. It should boot the game, so sit at the game's menu for a minute. Then, unplug the IDE and hotswap into your computer's XboxHDM (which should be running). Then, try to fix it.