If you have downloaded the free Red or Black packs for Rainbow Six Vegas, you may get a false disc error message. This is false in that the disc may not really be bad. You can fix your system and play the game again by deleting these packs from your Xbox.
Here is my experience: I downloaded both packs to my XBox. When I tried to play the game, the screen that there is an update to the game displays very briefly (under 1 second) and then shows a screen saying the disc is bad. Tried another disc... same problem. Tried another disc... same problem. Only had problem with Rainbow Six - all my other games worked fine. Rainbow Six Vegas 2 also worked fine.
I tried changing the boot settings - e.g. boot from disc or boot to XBox then load disc - and that made no difference.
However, as I was about to give up on playing the game and was about to delete my game information from my system (profile, add-ons, etc.), it occured to me to try deleting some of the game content on my system to see if this would fix the problem. I left my profile (since I'd worked so hard for that progress and those achievements), kept the videos (guessing they would have no impact), but deleted the Red and Black add-on packs. As soon as I did this, tried to play the game and everything worked just fine.
I started playing Rainbow Six Vegas, and probably downloaded these packs, before recent big update to the XBox interface. It is possible that re-installing the packs again may work. I haven't tried that.