I think its your lens/DVD drive. If there's a problem with it (or the disc) you can't install the disc. My copy of Tomb Raider Underworld is scarred to shit. It can't install. Like you said, it only got to 50%-60% before the 360 said, "fuck it. I can't install this shit."

Also, even if you have a game installed to the HDD, you have to have the actual disc to play the installed game. But you have the disc, you say? Well, if you happen to be downloading anything from Xbox Live, the 360 stops reading data from the HDD and starts using the disc, again. If there's a disc/disc drive problem then you're going to have problems. Were you downloading at the time?

Hope I helped ya. But I guess telling you that your disc drive is probably fucking up isn't really helping, is it?

I went through 3 copys of RB6 V2 and none of them worked... Im sure its my Xbox.

Luckly i still got my 64mb memory card.