So my wife has not used her computer for a few months because she has been out of school and has primarily been using her iPad or my computer. It's literally been months since her computer has booted up. She tried to start it the other day and it wouldn't go past the grey apple loading screen. I got it to boot in SAFE mode but can't get it to restart. I do the Command R thing and tried to reinstall the OS X (she was still using Lion) but it says the disk is locked. When I go into the Disk Utility menu and try to verify and repair the disk it says "Disk Utility can't repair this disk. Back as many of your files as possible, reformat the disk, and restore your backed-up files". Problem is that my wife never backed her computer up when I told her to and I can't get it backed up at this point. She has like 10 classes of school work that she needs for med school so she can't afford to loose her documents. I tried to install OS X on the external HD but it said "disk doesn't use the GUID Partition Table scheme" and it tells me how to change it. when I follow the instructions to change it I'm told the disk cannot be modified. Please Help!!!!!!!!!!!!!