If you have a Xbox 360 Slim Arcade and have tried to play Halo: Reach in co-op, then you've been met with the tragic error message that "One or more players do not have the hard drive required for this game type." The built-in 4 GB hard drive doesn't qualify as a "certified hard drive", which Microsoft wants you to have to play the game. Bummer!
Luckily, this is a bug and not a feature and they are working on a fix that should be available shortly.