Boot the Mac into Single User Mode by holding down Command+S during system boot after you hear the boot chime, you know you will have successfully entered Single User Mode because you will see a bunch of white text on a black background scroll by
When the Single User boot sequence has finished, you’ll find a small command prompt at the bottom of the screen prefixed by a hash sign (#), when you see that type the following command exactly:
fsck -fy
Once fsck completes, if you see a “File system was modified” message, then you should run “fsck -fy” again until you see a message stating “The volume (name) appears to be OK” – this is standard procedure of using fsck
Type “reboot” to leave Single User Mode.