Problem: XBox games aren't saving after XBox upgrade. I ran through the new XBox experience upgrade a couple days ago, then found that my Halo 3 games weren't saving. I'd have to completely restart the campaign after shutting down the console. Restores to the last checkpoint were working fine when I'd die in the session, but starting a new session reset me back to square one. Sucky.
Solution: Clear the XBox disk cache. Go to the system blade, then memory, then drill down to your hard drive. Press X, X, Left Bumper, Right Bumper, X, X. You'll be asked if you want to perform maintenance on the drive. Answer yes. The system will clear cache from the disk which solves the problem. NOTE: All game updates will be deleted, which means you'll have to reload them, but that's only a tiny hit for fixing the problem!