OS-X – file system check – fsck in Single User (teminal) mode

1. Boot the computer, as soon as you heat the chime, hold down both the COMMAND and S keys.
2. Terminal like interface will appear, wait until the boot process completes and the scrolling of text stops.
3. Press Enter twice to ensure all boot processes have completed.
4. Enter the following command, which will run the file system check (and repair) followed by a reboot – and you are back to work.

   fsck -fy && reboot
   

 

References: OS-X / Mac boot options list and descriptions!