My brother's MacBook Pro had a similar issue a few years ago. In his case the issue started when he attempted to update the OS to the then most recent version. The update failed with an error saying that the Hard Disk needed to be repaired and instructing him to restart in Recovery Mode and repair the disk. When he did this the "Repair Disk" option was grayed out as you said. From my searches then it seems that the drive entered a "write-locked" state that prevented it from mounting. I was able to recover his files and restore the computer to working order. You will need another computer that has enough free space to hold the files you want to save, and the appropriate cable to connect the misbehaving computer to the working computer. This procedure requires booting in Recovery Mode and erasing the non-working computer's hard drive, reformatting it, and restoring the system. 1. Turn on the working computer. 2. Make sure the non-working computer is turned off, connect the two computers using the appropriate...