Using Disk Utility to Repair Hard Drives and Disk Permissions

By Tom Nelson

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Use Disk Utility’s First Aid to Repair Drives and Disk Permissions

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Disk Utility First Aid - Screen shot courtesy of Coyote Moon, Inc.

The First Aid tab is where you will find Disk Utility's repair tools. Photo Credit: Screen shot courtesy of Coyote Moon, Inc.

Updated July 28, 2016.

The Disk Utility app has long been included with OS X for working with a Mac’s storage devices, including hard drives, SSDs, CDs, DVDs, flash drives, and more. Disk Utility is very versatile, and not only can 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, as well as repairing drives that are exhibiting various types of issues, including those that may cause a Mac to fail during startup or freeze while being used.

Two Versions of Disk Utility: Which Is the Right One for You?

Disk Utility has evolved over time, gaining new features with each new version of OS X. For the most part, Apple simply added on features and capabilities to the original Disk Utility core app. When OS X El Capitan was released, Apple decided to create a new version of Disk Utility. While it retains the same name, its user interface underwent a dramatic makeover. As a result, we've created two separate guides for working with Disk Utility's First Aid feature.

Using First Aid With OS X El Capitan and Later

If you're using OS X El Capitan, or macOS Sierra and later, you should jump over to the Repair Your Mac's Drives With Disk Utility's First Aid article to see the instructions for the First Aid feature matched up to the correct version of Disk Utility.

Using First Aid With OS X Yosemite and Earlier

If you're using OS X Yosemite or earlier, you're right where you need to be. This document will guide you through the process of using Disk Utility's First Aid feature for the version of OS X you're using.

First Aid Features

Disk Utility’s First Aid feature provides two unique functions. One can help you repair a hard drive; the other lets you repair file and folder permissions.

Repair Disk

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 at making minor disk repairs to a volumes file system, and it can make most repairs to a drive’s directory structure, but it’s no substitute for a good backup strategy. The Repair Disk feature is not as robust as some third-party applications that do a better job of repairing drives as well as recovering files, something Repair Disk is not designed to do.

Repair Disk Permissions

Disk Utility’s Repair Disk Permissions feature is designed to restore file or folder permissions to the state the OS and applications expect them to be in. 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 are 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.

What You Need

Disk Utility, located at /Applications/Utilities/.
Ten minutes to an hour of your time, depending on which First Aid tools you will be using.

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Using Disk Utility to Repair Drives and Volumes

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Disk Utility Repair Disk - Screen shot courtesy of Coyote Moon, Inc.

After a successful repair, Disk Utility won't display any error or warning messages, and will display green text specifying the volume is OK. Photo Credit: Screen shot courtesy of Coyote Moon, Inc.

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’ button will be grayed out. You will only be able to use the Verify Disk feature, which can examine the drive and determine whether anything is wrong.

Repairing a startup drive with Disk Utility is still 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.

Aside from the time required to restart from another hard drive an installation DVD or the Recovery HD, using Disk Utility’s Repair Disk feature otherwise works the same way and should take about the same amount of time. If you need to boot from an OS X installation DVD, you’ll find instructions on how to do this on pages 2 and 3 of Installing OS X 10.5 Leopard: Upgrading to OS X 10.5 Leopard. Start the process on page 2 of the guide, at the heading, “Start the Process: Alternative Method.”

Repair Disk

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 Repair Disk’s attempt to scan and repair it kicked the drive over the edge.

Launch Disk Utility, located at /Applications/Utilities/.

Select the ‘First Aid’ tab.
In the left-hand pane, select the hard drive or volume you wish to run Repair Disk on.
Place a check mark in the ‘Show details’ box.
Click the ‘Repair Disk’ button.
If Disk Utility notes any errors, repeat the Repair Disk process until Disk Utility reports ‘The volume xxx appears to be ok.’

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Using Disk Utility to Repair Permissions

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Repairing Disk Permissions routinely results in many warnings about permissions that differ from the expected.

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 will suggest running Repair Permissions. Luckily, Repair Permissions is pretty benign. Even if your Mac doesn’t need any permissions fixed, Repair Permissions is unlikely to cause any type of problem, so it remains one of those things to do “just in case.”

With the advent of OS X El Capitan, Apple removed the Repair Permissions functionality from Disk Utility. The reason behind the move is that starting with OS X El Capitan, Apple has started locking down system files, preventing permissions from being changed in the first place. Even so, whenever the operating system is updated, the permissions of the system files are checked and repaired, if needed, automatically.

When to Use Repair Permissions

You should use Repair Permissions if you're using OS X Yosemite or earlier, and you experience a problem with an application, such as an application not launching, starting up very 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 Actually Fixes

Disk Utility’s Repair Permissions only repairs files and applications that are installed using Apple’s installer package. Repair Permissions will verify and repair, 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 will only verify and repair files located on bootable volumes that contain OS X.

To Repair Permissions

Launch Disk Utility, located at /Applications/Utilities/.
Select the ‘First Aid’ tab.
In the left-hand pane, select a volume you wish to run Repair Permissions on. (Remember, the volume must contain a bootable copy of OS X.
Click the ‘Repair Disk Permissions’ button.
Disk Repair will list any files that don’t match the expected permission structure. It will also attempt 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.