This just happened on my new rmbp and also happened on my old 2011 mbp. I have about 136 gigs of data in my apps, library etc.... The Mac HDD however shows I'm using about 270 gigs. WTF? I got the info for all my apps, libraries, system, users etc.. and they indeed total out to the proper amount of 136 gigs. Unfortunately my first attempt at Time Machine failed last night and I have no backup. I manually backed up everything except MacHDD. Can I delete complete HDD, reinstall Yosemite and reinstall all my backed up files?