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Updated July 18, 2016.
Time Machine is an easy-to-use backup system with a nice collection of features that make it the go-to backup system for most Mac users. But like all backup applications, Time Machine is subject to errors and problems that can creep in and cause you to worry about your backups.
One of the common problems you may come across is Time Machine being unable to access the backup disk. The error message is usually:
The good news is that your backup files are probably all in good working order and no backup data has been lost. The bad news is that you can't backup any new data to your Time Machine drive until you get this problem fixed.
The cause of the error message depends on a few factors, but in all cases, your Mac thinks the drive has had its permissions changed to be read only. But don't head off and try to reset the permissions because it won't do you any good. Instead, follow these simple steps.
Launch System Preferences, and select the Time Machine preference pane.
Move the slider to OFF.
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 the drive or restarting your Mac. While I can't tell you the reason, I can tell you that this is by far the most common solution for the "backup volume is read only" error.
- If your Time Machine drive is mounted on your desktop, right-click the drive and select Eject "drivename" from the pop-up menu. Jump to step 4.
- If your Time Machine drive isn't mounted on your desktop, launch Disk Utility, located at /Applications/Utilities.
- Select the Time Machine drive from the Disk Utility sidebar, and then click the Unmount button in the toolbar.
- Once the drive is ejected, you can turn it off or disconnect its cable.
- Wait 10 seconds, then plug the drive back in and turn the power to the drive back on.
- The drive should mount on your desktop.
- Turn Time Machine back on by launching System Preferences, selecting the Time Machine preference pane, and moving the slider to ON.
- Time Machine should be able to use the drive once again.
- If Time Machine still can't access the drive, proceed to the next step.
If your Time Machine drive isn't an external volume connected directly to your Mac, or 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.
- Turn Time Machine off.
- Use Disk Utility's ability to repair minor drive issues to correct the read-only problem; you'll find instructions in this guide:
- 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).
- Once the drive is repaired, turn Time Machine back on. It should now be able to use the drive.
If you're using a Time Capsule, you can use the following instructions to repair the drive.
- Mount your Time Capsule on your Mac's desktop.
- Open a Finder window and locate your Time Capsule in the Finder window's sidebar.
- Double-click your Time Capsule to open it in a Finder window.
- In the Time Capsule window, open the Backups folder.
- Within the Backups folder, you'll find a file whose name ends in .sparsebundle.
- Drag the .sparsebundle file to the sidebar of the Disk Utility app.
- Select the .sparsebundle file in the Disk Utility sidebar.
- Click the First Aid tab.
- Click the Repair Disk button.
- Once the repair is complete, you can close Disk Utility.
- Turn Time Machine back on. It should now be able to use your Time Capsule.
The short answer is yes; in most cases, this one-time problem is unlikely to have any effect on 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 problems that require you to use Disk Utility or a third-party drive utility app to repair the drive, then you will be fine. In all likelihood, this was a one-time event, perhaps caused by a power outage, or your Mac or the Time Machine drive turning off unexpectedly.
As long as the problem doesn't repeat, your Time Machine drive should be in good shape. However, if the problem continues to reoccur, you may want to consider a new drive to store your precious backups.
You may also want to look at:
Reviving a Hard Drive for Use With Your Mac
Originally published: 10/17/2013
Update history: 4/1/2016, 7/18/2016
Source: Tom Nelson, http://macs.about.com/od/Troubleshooting/qt/Fixing-Time-Machine-Errors-The-Backu p-Volume-Is-Read-Only.htm
