Hello everyone, I have a 2013 27" iMac with a 1TB Fusion drive in it. Yesterday I was just browsing the internet on Safari when all of a sudden my computer just froze up. I thought it was just Safari and all it's snippyness but I couldn't even force quit it. So I just held down the power button till it shut off. When I turned the computer back on, it just displayed a flashing folder with the question mark on it. I though ok? No big deal, I have my time machine backups. So I booted into the recovery mode and went into disk utility and erased the Mac OS X partition. After that I then attempted to restore my system from the latest time machine backup. It said it could not do that and to restart my computer and try again. I got the same results. So I then tried to install a clean copy of 10.9 and got the same message. THEN tried internet recovery with the SAME message... I booted back up into recovery mode (all this time running off my time machine backups) and do into disk utility again. I clicked on verify to verify the Macintosh HD Not the Mac OS X partition. I came up with the error message "invalid b-tree node size. This disk needs to be repaired." So I click repair and it just says the same thing. Disk utility can not repair the disk. So I don't know what is going on? I guess my hard drive it just completely shot? Basically my question is this... I do have access to the terminal. Is there anyway I can completely reformat the Macintosh HD? I do mean the Macintosh HD. Not erase the Mac OS X partition even at the most secure leave because that does not work. Either way I have a Genius Bar app. Friday, but any help is very much appreciated! Thank you for your time! After talking to someone at the Genius Bar, all they did was dump the image for Mavericks on my HD. Then did NOT solve my issue. My disk needed to be repaired.