Fixing Time Machine Errors - Backup Volume Is Read Only

How to fix a Time Machine backup that failed with a read-only error

Hard drive for backing up data

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Time Machine is an easy-to-use backup system with a collection of features that make it the go-to backup system for most Mac users. However, like all backup applications, Time Machine is subject to errors and problems that can creep in and cause you to worry about your backups.

Common Time Machine Backup Errors

One of the common problems you may encounter occurs when you see this error message:

The backup volume is read-only

Despite this message, your backup files are probably in good working order, and no backup data has been lost. However, you can't back up any new data to a Time Machine drive until this problem is fixed.

The cause of the error message depends on a few factors, but in most cases, the Mac thinks that the drive permissions have changed to read-only.

When using an external drive connected to a Mac by USB, FireWire, or Thunderbolt, eject the drive from the Mac, then reconnect the drive and restart the Mac. This is the most common solution for the backup volume is read-only error.

If ejecting and reconnecting an external drive doesn't help, don't reset the permissions because it won't do any good. Instead, follow these troubleshooting steps.

Turn Off Time Machine

  1. Go to the Apple menu, then select System Preferences.

  2. In the System Preferences window, select Time Machine.

    System Preferences menu and Time Machine icon in macOS
  3. Clear the Back Up Automatically check box to turn off Time Machine.

    Unchecked
  4. If the Time Machine drive is mounted on the desktop, right-click the drive and select Eject [drivename]. If the Time Machine drive isn't mounted on the desktop, go to Applications > Utilities, then select Disk Utility.

    Over the years, Apple has made two similar versions of Disk Utility. The one illustrated here is included with macOS Mojave. More information about Disk Utility is included in Using Disk Utility to Repair Hard Drives and Disk Permissions (OS X Yosemite and earlier) or in Repair Your Mac's Drives With Disk Utility's First Aid (OS X El Capitan and later).

  5. In the Disk Utility window, select Time Machine, then click Unmount.

    Unmount button in Disk Utility/Time Machine Backups
  6. Once the drive is ejected, turn it off or disconnect its cable.

  7. Wait 10 seconds, then plug in the drive and turn on the power on the drive. The drive should mount on the desktop.

  8. Turn Time Machine back on. Open System Preferences, select Time Machine, then select the Back Up Automatically check box.

Time Machine should be able to use the drive once again. If Time Machine still can't access the drive, repair the drive.

Repair a Time Machine Drive

If the process outlined above didn't correct the problem, then it's likely that the Time Machine volume has disk errors that need to be repaired. Here's how:

  1. Turn off Time Machine. Go to System Preferences > Time Machine, then clear the Back Up Automatically check box.

  2. Use Disk Utility to repair minor drive issues to correct the read-only problem. In the Disk Utility screen, select Time Machine, then click First Aid.

  3. Click Run to confirm that you want to run First Aid on Time Machine.

    Run Button for Disk Utility First Aid in macOS
  4. Wait while Disk Utility works on the Time Machine drive. This process usually takes several minutes. The progress appears in the window along with the details on what is being checked or repaired.

    Disk Utility First Aid progress screen
  5. When the repairs are complete, you may no longer encounter the Backup Volume Is Read-Only error message.

  6. Exit Disk Utility and turn on Time Machine.

Is It OK to Use a Drive That Needed Repairs for Time Machine?

In most cases, this one-time problem is unlikely to have any effect on the reliability of your Time Machine drive. As long as the Time Machine drive doesn't continue to have problems that require Disk Utility or a third-party drive utility app to repair the drive, then you should be fine. In all likelihood, this was a one-time event, perhaps caused by a power outage or the Mac or the Time Machine drive turning off unexpectedly.

As long as the problem doesn't repeat, the Time Machine drive should be in good shape. However, if the problem continues to occur, it may be time to consider a new drive to store your backups.