Fixing a Time Machine backup that failed with a read-only error

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

One of the most common issues you may encounter is Time Machine not being able to access the backup disk. The error message is usually:

“The backup volume is read-only”

The good news is that your backup files are probably in good working order and no backup data has been lost. The bad news is that you can’t back up any new data to your Time Machine drive until you resolve this issue.

The cause of the error message depends on a few factors, but in all cases, your Mac thinks the drive’s permissions have been changed to read-only. But don’t go astray and try to reset permissions because it won’t do you any good. Instead, follow these simple steps.

Disable Time Machine

  1. Launch System Preferences and select the Time Machine preferences pane.
  2. Move the slider to the OFF position.

external drive

If you’re using an external drive connected to your Mac via USB, FireWire, or Thunderbolt, you can try ejecting the drive from your Mac and then reconnecting it or restarting your Mac. While I can’t tell you why, I can tell you why. is by far the most common fix for the “backup volume is read-only” error.

  1. If your Time Machine drive is mounted on your desktop, right-click the drive and select Eject “drivename” from the pop-up menu. Skip to step 4.
  2. If the Time Machine drive is not mounted on the desktop, run Disk Utility, located in /Applications/Utilities.
  3. Select the Time Machine drive in the Disk Utility sidebar, then click the Unmount button in the toolbar.
  4. Once the drive is ejected, you can turn it off or unplug its cable.
  5. Wait 10 seconds, then plug the unit back in and turn it back on.
  6. The unit must be mounted on the desktop.
  7. Turn Time Machine back on by launching System Preferences, selecting the Time Machine preferences pane, and moving the slider to ON.
  8. Time Machine should be able to use the drive again.
  9. If Time Machine still can’t access the drive, continue to the next step.

Repairing a Time Machine drive

If the Time Machine drive isn’t an external volume directly attached to your Mac, or if the process described above didn’t correct the problem, the Time Machine volume likely has disk errors that need to be repaired.

  1. Turn off Time Machine.
  2. Use Disk Utility’s ability to repair minor drive problems to correct the read-only problem; You will find instructions in this guide:
  3. Using Disk Utility to repair hard drives and disk permissions (OS X Yosemite and earlier) or repair your Mac’s drives with Disk Utility First Aid (OS X El Capitan and later) .
  4. After the drive is repaired, turn Time Machine back on. You should now be able to use the drive.

Repair a Time Capsule

If you are using a Time Capsule, you can use the following instructions to repair the drive.

  1. Mount your Time Capsule on the desktop of your Mac.
  2. Open a Finder window and locate your Time Capsule in the Finder window’s sidebar.
  3. Double-click your Time Capsule to open it in a Finder window.
  4. In the Time Capsule window, open the Backups folder.
  5. Inside the Backups folder, you will find a file whose name ends with .sparsebundle.
  6. Drag the .sparsebundle file to the sidebar of the Disk Utility application.
  7. Select the .sparsebundle file you want to archive in the Disk Utility sidebar.
  8. Click on the First Aid tab.
  9. Click the Repair Disk button.
  10. After the repair is complete, you can close Disk Utility.
  11. Turn Time Machine back on. You should now be able to use your Time Capsule.

Is it okay to use a repairable drive for Time Machine?

The short answer is yes; In most cases, this one-time issue is unlikely to affect the reliability of your Time Machine drive.

The long answer is a bit, well, longer.

As long as your Time Machine drive doesn’t continue to have issues that require you to use Disk Utility or a third-party drive utility app to repair the drive, then you’ll be fine. This was most likely an isolated event, perhaps caused by a power outage, or your Mac or Time Machine hard drive shutting down unexpectedly.

As long as the problem doesn’t recur, your Time Machine drive should be in good shape. However, if the problem keeps recurring, you may want to consider a new drive to store your valuable backups.

You may also want to look at:

Revive a hard drive for use with your Mac