Having trouble starting a co-op campaign game.

I recently bought a 4gig wifi xbox, I really enjoy the new design. but know as istarted to play halo reach co-op with a friend. was unable to start a co-op match. A message prompt " One or more players do not meet hard drive requirements." What a fustrating thing to accur specially after waiting so long for halo reach release. anyone have an idea whats going on?