Disk Utility 12.x: Repair a non-startup disk

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Repair a non-startup disk using Disk Utility

You might need to repair your disk if you experience certain problems with it—for example, an application quits unexpectedly, a file is corrupted, an external device doesn’t work properly, or your computer won’t start up.

This page describes how to repair a disk that you didn’t use to start up your Mac. If you’re having trouble with the disk that started up your Mac, follow these instructions:

Repair a disk

  1. Close files and quit all applications on the disk you want to repair.
  2. Open Disk Utility, in the Utilities folder in Launchpad.
  3. In the list at the left, select the item you want to repair. (Be sure to select an item that’s indented to the right in the list, not an item at the far left.)
  4. Click First Aid.
  5. If Disk Utility tells you the disk is about to fail, back it up and replace it. You can’t repair it.
  6. Click Repair Disk.

    If Disk Utility can’t unmount the disk, be sure you have quit any open applications, and click Repair Disk again. If it still can’t unmount the disk, follow the steps for repairing a startup disk.

    If Disk Utility reports that the disk appears to be OK or has been repaired, you’re done. Otherwise, you may need to do one of the following steps.

  7. 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. Check each file in the list of affected files. If you can replace a file or recreate it, delete it. If it contains information you need, open it and examine its data to make sure it hasn’t been corrupted. (Most of the files in the list have aliases in a DamagedFiles folder at the top level of your disk.)
  8. 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 Mac OS X, and then restore your backed-up data.

    If you continue to have problems with your disk, it may be physically damaged and need to be replaced.

Published Date: Sep 3, 2015
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