How to verify a Time Machine backup

We recently put out a call for topics you‘d like to learn more about and one of the most popular suggestions was how to verify a Time Machine backup. After all, you‘ve gone to all the trouble to create such a backup, it would be nice to know if it works. It turns out that there‘s not a completely clear-cut answer to this one. Let me explain.

I‘ll open the Time Machine preference and ensure that the Time Machine menu appears in the menubar. I‘ll then hold down the Option key and click on this menu and, look at that, there‘s a Verify Backups command. But wait, it‘s grayed out.

It‘s grayed out because this is a Time Machine backup housed on a drive connected directly to my Mac. This command works only for networked Time Machine backups—either backups on an Apple Time Capsule, or a networked drive on another Mac or an NAS.

Through the miracle of editing I‘ve switched my Time Machine backup to a drive I have attached to my Mac mini and the command is now active. At this point I can choose to verify the backup.

But many people don‘t have a network backup device. What are they supposed to do? First, you can do the obvious thing and try restoring a file using Time Machine. If it works, that‘s a very good sign.

You could then access the contents of your Time Machine backup and see if real files appear. Try copying one. If it works this is another good sign.

If you really want to get down to it, launch Terminal and enter this command: tmutil compare and then press Return.

When you do this, tmutil will run and compare the contents of the last Time Machine snapshot with the current state of of your Mac. Entries with a plus next to them tell you that thing is new. Files with a minus tell you that thing has been removed. And an exclamation mark indicates that a file has changed.

To compare the amount of data that‘s changed, enter tmutil compare -s. If you see zeros after the Added, Removed, and Changed entries, this tells you that the backup is in sync with your Mac and is very likely working as expected.

If you suspect that your Time Machine backup is wonky, you can verify it and then try to repair it. To do that, open Time Machine‘s preferences and switch off Time Machine. Then launch Disk Utility, select the drive you normally use for your backup, and then click the Verify button, just as you‘d do with any other drive you want to check.

Given the uncertainty that surrounds Time Machine verification it makes a whole lot of sense to have more than one backup. While many people create a Time Machine backup and then use another utility to clone their drive, you can create two Time Machine backups easily.

Just go into Time Machine‘s preference and select an additional disk and choose to use it as a backup. You‘ll be presented with the option to use both disks. Do so. Now, when Time Machine performs future backups it will back up to the first disk, and on the next go around, back up to the next one.

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