So i have a 27inch imac shipped with mountain lion. Once i turn it on it starts to boot then I see a progress bar (like the one for safe boot but i am not holding the shift key or any key), then it would stall at quarter way and then restart or shut down. I booted into the recovery partition and attempted to repair the disk but it said disk utility cannot repair this disk and i should backup and restore the machine. I tried single user mode with fsck but preety much the same error and exited with signal 8. I know the hdd is not defective as it was tested at an authorized apple location. i also know a restore will fix this as I had this same problem a few days ago an a different mac and formatting, partitioning fixed it. I can restore the system now that I found the files i needed backed up somewhere else on the cloud. However did anybody get this before or know the reason, solution?