Bad news! Owners of the new slim Xbox 4GB consoles cann't play "co-op campaign mission" in Halo: Reach, which's the large part of the Halo: Reach experience. According to FAQ on Halo: Reach's Xbox page:
Q1: The error "One or more players don't have an Xbox 360 Hard Drive. An Xbox 360 Hard Drive is required to play co-op on Xbox LIVE or system link" occurs when I try to play. Why?
A1: One or more players don't have a certified Xbox 360 hard drive attached to their Xbox 360 console. Some multiplayer aspects of Halo: Reach require an Xbox 360 certified hard drive.
Us gamers can buy a Xbox 360 250GB HDD for $129 from Amazon.