With the advent of OS X El Capitan, Apple made a few changes to how the Disk Utility First Aid feature works. The main difference is that First Aid will verify the selected drive and automatically attempt to correct any problems. Before El Capitan, you could just run the Verify process on its own and then decide if you wanted to attempt repairs.
Disk First Aid and the Startup Drive
You can use Disk Utility’s First Aid on your Mac’s startup drive. However, you’re limited to only performing a verification of the drive while the operating system is actively running from the same disk. If there’s an error, First Aid will display it but won’t attempt to repair the drive. To get around the problem, start up from your Recovery HD volume or another drive with a bootable copy of the operating system installed. The two methods are similar; the main difference is the need to boot from another volume instead of your normal startup drive.
First Aid From a Non-Startup Volume
Here’s how to use Disk Utility’s First Aid on a non-startup volume.
Launch Disk Utility
Use Spotlight (Command + Spacebar) to Launch Disk Utility or find it from /Applications/Utilities/. The Disk Utility window appears as three panes:
Button bar: Across the top of the window is a button bar containing commonly used functions, including First Aid.Mounted volumes: On the left is a sidebar that displays all the mounted volumes connected to your MacMain pane: On the right is the main pane, which displays information from the currently selected activity or device.
Select the Volume
Use the sidebar to select the volume you wish to run First Aid on. The volumes are the items just below a device’s primary name. As an example, you may have a Western Digital drive listed, with two volumes below it named Macintosh HD and Music. The right pane displays information about the selected volume, including size and amount of space used.
Run First Aid
With the volume you wish to verify and repair selected:
Repairing Drives
Some notes on what to expect when using First Aid to repair a drive:
If First Aid reports no issues: If First Aid indicates that the drive appears to be okay or that repairs are complete, you’re done. In some previous versions of First Aid, it was necessary to run the repair process multiple times to ensure the repairs were complete; that is no longer required. If First Aid displays an “overlapped extent allocation” error: Disk Utility will create a DamagedFiles folder at the root level of your startup drive. The overlapped error indicates that two (or possibly more) files occupied the same location on the drive receiving repair. More than likely, both files have become corrupt, but there’s a slight chance you can recover one or both of them. You can examine the files in the DamagedFiles folder. If you don’t need the file, or you can delete the file and easily recreate it. If you must have the file, then check your backup for a usable copy. If First Aid reports a failure: “The underlying task reported failure” message indicates that it failed at making the needed repair. However, don’t give up; try rerunning the repair a few times. If repairs aren’t successful: As long as you have a backup of the data stored on the affected drive, reformat the drive and perform a clean installation of your operating system version. You can then restore your backup data using the Migration Assistant.
Boot From the Recovery HD
To use the Recovery HD method, use these complete step-by-step instructions to boot from the Recovery HD volume and start Disk Utility. Once you have successfully restarted from the Recovery HD and have launched Disk Utility, you can use the method for using First Aid on a non-startup drive to verify and repair the drive.
Additional Guides That Can Help With Drive Problems
If you need more help with your Mac’s drives, consult these other step-by-step and troubleshooting guides to using the Mac’s Safe Boot option or repairing your hard drive when your Mac won’t start.
