Get a PC. Problem solved.
I was hoping for no PC fanboys in this thread... :(It wont let me defragment because i need 15% space... what?
Chances are, you have a 20 GB movie file of you playing Ratchet and Clank hidden somewhere lost in a folder.At least, this happened to me once. Warren won't let me live it down...
Delete all the research photos/videos in your secret hidden folder.