Start up in single user mode, which is a minimal BSD Unix environment. You can run /sbin/fsck -fy from the CLI, which will force-correct any disk problems. If you get a message that "FILE SYSTEM WAS MODIFIED" this means fsck found and repaired problems. Run it again until no error messages show: i.e.: ** The volume (name_of_volume) appears to be OK **. Exit single user mode by typing reboot.