Answer to: 15" MacBook Pro won't boot upGray screen and spinning icon is surely gray screen of death. Try disconnecting and reconnecting peripherals & re-start your Mac. But you must also not rely on a failing hard drive if this is the case with you. I suggest you to image it rather than clone because image will skip all the bad sector of the drive while clone will copy everyting including the bad sectors. You can try feature of http://www.stellarinfo.com/mac-software/clone-mac/clone-mac-drive.php to image your failing hard drive.