- Make a backup of your Mac.-Even if you're repairing a disk on your Mac, if the repair fails, your disk may be in worse shape than it was before you started. As a result, we strongly advise you to back up all of your data on your Mac before you begin.
- Enter Recovery Mode on your Mac.-If you need to repair your Mac's starting disc, you must first boot into Recovery Mode. This is due to the fact that you can't repair the startup disc when your Mac is running macOS.
- In Disk Utility, select a volume, container, or disc from the sidebar. Before moving on to the containers and the disc, start with the bottom volume and work your way up.
- Click the First Aid button at the top of the Disk Utility and select Run. If there isn't a Run button visible, select Repair Disk instead. It's possible that you'll need to enter your administrator password.
- Click Done when First Aid is finished, and then repeat the process for the next volume, container, or disc.
- When you've finished repairing all of your Mac's discs, go to Apple > Restart in the menu bar and restart your computer.
Read More: How to fix a Mac that won’t start up
