I was just playing a game for 2 hours from a backup dvd (Verbatim Dual Layer), when, during a loading screen, the damaged media message popped up. I tried a few times more, but then I looked at the dvd and noticed several circular scratches very near to each other, forming a ring (maybe 2mm thick) about halfway between the center and the rim. Now the game plays only for a short time, mostly it doesn't get past the start menu.
So, my first guess is that I have a damaged drive. The other DVDs I played today on the Xbox haven't suffered any damage though.
Has anyone else experienced something like it?
I'm making another backup right now to find out if I have to get another drive.