1. Press the power button to start the computer.
2. Immediately press and hold the Command (Apple) key and the "s" key for single-user mode.
At the command-line prompt, type: /sbin/fsck -fy
The fsck utility will go through five "phases" then return information about the disk's utilization and fragmentation. Once the check is finished, if no issue is found, you should see "** The volume (name of volume) appears to be OK."
If fsck alters, repairs, or fixes anything, it will display the message:
***** FILE SYSTEM WAS MODIFIED *****
Important: If this message appears, repeat the fsck command until it no longer appears. It's OK if you need to do several "passes" of fsck, because first-pass repairs may uncover additional issues.
When fsck reports that, "** The volume (name of volume) appears to be OK.", type: reboot