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Time Machine backups are a great safety net. You should always create a Time Machine backup before making any changes at the operating system level, such as installing a beta version of macOS, and before applying any experimental or unsupported changes, such as using a third party app to alter your system font.
However, it is possible for Time Machine backups to become corrupted – this can happen naturally over time, but it can also happen maliciously, for example a hacker may use your Time Machine backups as a way of getting viruses onto your Mac.
Time Machine has a built-in feature that can help you identify whether a backup has changed since it was created. This ‘verify backups’ feature compares the checksum generated when the backup was created, to the current checksum – if these digits are different, then this is a sign that the backup has become corrupted, or been modified in some way.
In this article, I’ll be showing you how to use this feature to verify the integrity of any backup made to a server, AirPort Time Capsule, or a drive that’s either connected directly to your Mac, or to an AirPort Extreme Base Station. This feature works with both encrypted and unencrypted backups, and for all backups created after macOS 10.11.
While you’ll typically use this feature to verify the integrity of a backup before restoring it, this feature can also be handy if you encounter problems with restoring a particular Time Machine backup, as it gives you a way of testing whether this problem is being caused by the backup itself.
Verify the integrity of any Time Machine backup
The easiest way to verify the integrity of a backup, is to use the Time Machine icon in your Mac’s menu bar. If you haven’t added this icon to your menu bar, then do so now:
- Click the ‘Apple’ logo in the menu bar.
- Select ‘System preferences > Time machine.’
- Select the ‘Show Time Machine in menu bar’ checkbox.
Then, to compare the checksums:
- Make sure the backup you want to verify is currently available, for example if it’s stored on an external drive then make sure this drive is connected to your Mac.
- Select the Time Machine icon in your Mac’s menu bar.
- Press and hold down the ‘Option’ key on your keyboard.
- Select ‘Verify Backups’ from the dropdown menu that appears.
Time Machine will then run its checksum test, and display a dialogue if it discovers any discrepancies between these two checksums.
Verifying backups from the Terminal
If you’re comfortable with issuing Terminal commands, then you can also verify the integrity of a backup using macOS’ built-in tmutil utility:
- If you backup to an external drive, then make sure this drive is currently available.
- Launch your Mac’s Terminal, by opening a Finder window and navigating to ‘Applications > Utilities > Terminal.’
- Type the following command into the Terminal:
tmutil verifychecksums /path-to-your-backup
- Press the ‘Enter’ key on your keyboard.