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It does make you update straight from the disc. I had this problem with Hitman: Blood Money, I backed it up, burned it, put it in the DVD tray, began to boot up when the 360 OS popped up and said something like "You must update your Xbox 360 before you can play this game..". I updated it, then it said this disc was dirty. I'm guessing that MS is putting countermeasures to the firmware hacks straight onto new games, so any new hacked firmware will be overidden by MS.
My sammy works fine even though I accept the updates on all of my games(including hitman), so I think its just a media/burner issue. Dont you think ms would crank up some fancy warning screen instead of dirty disc error if backup is detected?
"You are using an unauthorized copy of this game. You are a bad bad bad person, and are gonna rot in hell" etcetc