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Turn off your Mac by pressing and holding the power button for a few seconds.
If you don't see your startup disk in the Startup Disk window, close the window and try the next sections of this article.
If you don't see your startup disk
If you don't see your startup disk in the Startup Disk window, use these steps to see if your startup disk needs directory repair.
Choose Apple menu > Restart. Once your Mac restarts and you hear the startup chime, hold down the Command and R keys.
When the Apple logo appears, you can release the keys.
Click Disk Utility, then click Continue.
In the sidebar, select the disk you want to repair.
Click the First Aid button .
If Disk Utility tells you the disk is about to fail, back up your data and replace the disk. You can’t repair it. Otherwise, continue to step 6.
If Disk Utility reports that the disk appears to be OK or has been repaired, you’re done. You can click Show Details to see more information about the repairs. Otherwise, you may need to do one of the following.
If Disk Utility reports “overlapped extent allocation” errors, two or more files occupy the same space on your disk, and at least one of them is likely to be corrupted. You need to check each file in the list of affected files. Most of the files in the list have aliases in a DamagedFiles folder at the top level of your disk.
If you can replace a file or re-create it, delete it.
If it contains information you need, open it and examine its data to make sure it hasn’t been corrupted.
If Disk Utility can’t repair your disk or it reports “The underlying task reported failure,” try to repair the disk or partition again. If that doesn’t work, back up as much of your data as possible, reformat the disk, reinstall macOS, then restore your backed-up data.
If your Mac has a Fusion Drive and you see a flashing question mark or alert, see the troubleshooting section of the Apple Support article About Fusion Drive.
If you continue to have problems with your disk or it can’t be repaired, it may be physically damaged and need to be replaced.
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