Using Disk Utility to Repair Hard Drives and Disk Permissions

Use these tips for Disk Utility in OS X Yosemite and earlier

The Disk Utility app has long been included with OS X for working with Mac's storage devices, including hard drives, SSDs, CDs, DVDs, and flash drives. Disk Utility is versatile and erase, format, partition, and work with disk images. It's also the first line of defense when it comes to verifying whether a drive is working correctly, and it repairs drives that exhibit other types of problems, including those that cause a Mac to fail during startup or freeze while being used.

Information in this article applies to Disk Utility on Macs running OS X Yosemite (10.10) through OS X Lion (10.7).

Which Version of Disk Utility Is Right for You?

Disk Utility has evolved, gaining new features with each new version of OS X. For the most part, Apple added features and capabilities to the original Disk Utility core app. However, when Apple released OS X El Capitan, it created a new version of Disk Utility. While it retains the same name, its user interface underwent a dramatic makeover. Therefore, there are two separate workflows for using Disk Utility's First Aid feature.

If you use macOS Catalina (10.15) through OS X El Capitan (10.11), your version of Disk Utility differs from the one shown here. Jump over to Repair Your Mac's Drives With Disk Utility's First Aid to see the instructions for the First Aid feature in your version of Disk Utility.

First Aid to Repair Drives and Disk Permissions

If you're using OS X Yosemite or earlier, you're right where you need to be. Disk Utility’s First Aid feature provides two unique functions. One repairs a hard drive, while the other repairs file and folder permissions.

Disk Utility First Aid

Repair Disk Function

Disk Utility can repair common disk issues, ranging from corrupt directory entries to files left in unknown states, usually from power outages, forced restarts, or forced application quits. Disk Utility's Repair Disk feature is excellent for making minor disk repairs to a volume's file system, and it can make most repairs to a drive's directory structure. Still, it's no substitute for a backup strategy. The Repair Disk feature is not as robust as some third-party applications that recover files—something Repair Disk is not designed to do.

Repair Disk Permissions Function

Disk Utility's Repair Disk Permissions feature is designed to restore file or folder permissions to the state the OS and applications expect. Permissions are flags set for each item in the file system. They define whether an item can be read, written to, or executed. Permissions are initially set when an application or group of files is installed. The installation includes a .bom (Bill of Materials) file that lists all the files that were installed and what their permissions should be set to. Repair Disk Permissions uses the .bom file to verify and repair permission issues.

How to Repair Drives and Volumes

Disk Utility's Repair Disk feature can work with any drive connected to your Mac—except the startup disk. If you select the startup disk, the Repair Disk tab is grayed out. You are only able to use the Verify Disk feature, which examines the drive and determines whether anything is wrong.

However, repairing a startup drive with Disk Utility is possible. To do it, you must boot from another drive that has OS X installed, boot from the OS X installation DVD, or use the hidden Recovery HD volume included with OS X Lion and later.

Back up your drive first. Even though your drive is having some problems, it's a good idea to create a new backup of a suspect drive before running Repair Disk. While Repair Disk usually doesn't cause any new problems, it's possible for the drive to become unusable after an attempt to repair it. This isn't Disk Repair's fault. It's just that the drive was in such bad shape to begin with that the Repair Disk's attempt to scan and repair it kicked the drive over the edge.

To repair a drive using Disk Utility:

  1. Launch Disk Utility, located at Applications > Utilities.

  2. Select the First Aid tab.

  3. In the left pane, select the hard drive or volume you want to run Repair Disk on.

  4. Place a check mark in the Show details box.

  5. Click the Repair Disk button.

    Disk Utility Repair Disk
    After a successful repair, Disk Utility won't display any error or warning messages, and will display green text specifying the volume is OK.
  6. If Disk Utility notes any errors, repeat the Repair Disk process until Disk Utility reports The volume xxx appears to be ok.

How to Repair Permissions

Disk Utility's Repair Permissions may be one of the most overused services included with OS X. Whenever something isn't quite right with a Mac, someone suggests running Repair Permissions. Fortunately, Repair Permissions is benign. Even if your Mac doesn't need any permissions fixed, Repair Permissions is unlikely to cause a problem, so it remains one of those things to do “just in case.”

When to Use Repair Permissions

You should use Repair Permissions if you experience a problem with an application, such as an application not launching, starting up slowly, or having one of its plug-ins refuse to work. Permission problems can also cause your Mac to take longer than usual to start up or shut down.

What Repair Permissions Fixes

Disk Utility's Repair Permissions only repairs files and applications that are installed using Apple's installer package. Repair Permissions verifies and repairs, if needed, all Apple applications and most third-party applications, but it won't check or repair files or applications you copy from another source or the files and folders in your home directories. In addition, Repair Permissions only verifies and repairs files located on bootable volumes that contain OS X.

To repair permissions using Disk Utility.

  1. Launch Disk Utility, located at Applications > Utilities.

  2. Select the First Aid tab.

  3. In the left pane, select a volume you want to run Repair Permissions on. The chosen volume must contain a bootable copy of OS X.

  4. Click the Repair Disk Permissions button.

    Disk Utility - Using Disk Utility to Repair Permissions
    Repairing Disk Permissions routinely results in many warnings about permissions that differ from the expected.

Disk Repair lists any files that don't match the expected permission structure. It also attempts to change the permissions for those files back to the expected state. Not all permissions can be changed, so you should expect some files to always show up as having different permissions than expected.