Hello everyone, This is my first time posting here, and I'm not sure if this is the right place for this thread. However, any help would be greatly appreciated. I'm helping my brother with issues on his Mac since he is not very computer literate. He has a Mac Book Pro (2009), but he is not sure what OS. I know it is at least OS X 10.6.8, but he is unsure if he performed any updates afterwards. It recently started receiving the grey screen stop sign when booting. We tried following these instructions http://support.apple.com/en-us/TS2570. After attempting the safe mode boot, a flashing question mark was present. Now when holding option to boot, none of the startup drives are shown. A blank grey screen is the only thing that appears. We tried following these instructions as well http://support.apple.com/en-us/TS1440, but we cannot view any startup drives. I was able to create a bootable usb drive of OS X 10.10 and the installer loads up fine. The problem is only the flash drive shows up in disk utility and when attempting to install the OS. I tried a diskutil list in terminal, and the drives do not show up there either. However, if I reboot from this bootable OS X installer and hold option, all drives are now seen again on the startup screen(Mac OS X, Windows, Recovery HD, OS X Installer). In fact, I am even able to boot into the windows boot camp startup (partitioned from the same drive as Mac OS X). Therefore, I do not think that the HD is broken. However, trying to boot into Mac OS X or Recovery from here leads back into the grey stop sign. Any suggestions on how to get past this screen or how to get the drives to appear in order to reinstall OS X? I don't know what else to do at this point.