Found this on the gamefaqs/gamespot forum: http://www.gamespot.com/xbox360/action/brink/show_msgs.php?topic_id=m-1-59092744
Basically it looks like someone in the beta dashboard program have trouble running the game without installing it to the HDD first. The new dashboard is releasing next week so Splash Damage might want to investigate this and prepare a patch for it.
I'm talking about the dashboard update that's coming up next week. A few people got selected testing it for the past few weeks. It has nothing to do with game updates...