Here's the solution to this "problem." You simply delete, and then reinstall the new disc, thus eliminating the 18 gig spot. Don't tell me that a person can honestly sit and play the whole FFXIII straight through. If so, I hope they develop more social skills....
Everyone needs to play Lost Odyssey! Any opposition to this and I will have to just say, "If it's a fight you want, you got it!"