I don't know the percentage of owners who have just the 4GB version of the Xbox 360, but those that own Halo: Reach
as well won't be able to play the co-op campaign, as reported by Nukezilla
. If you try, an error message pops up saying one or more players do not have the hard drive required to play this game type.
Now, other modes are available and FAQ page
does say you need the hard drive to play the co-op campaign. Now, word is that Microsoft's looking into it so who knows, we might get a fix to this as adding more USB sticks doesn't help you play the game this way.