How to use multiple drives with Time Machine for redundant backups

When you create a backup system for your data, duplication is the best course of action. I don’t mean duplicating the files—that’s a requirement—but duplicating the destinations to which files are bound.

Every form of backup media is destined to fail, and despite high reliability from cloud-backup services, you can’t put all your faith that any of them will always be perfect. Even a system with “five nines” of reliability (99.999 percent) may suffer a loss, and the universe might pick you to experience that loss.

The rule of thumb is summarized as 3-2-1: three copies of your data, two of them local, and one offsite. One copy is your live version on your active drives; one can be a Time Machine backup; and the offsite one can be either a cloud backup, or your files stored somewhere securely and regularly rotate with the local backup.

Time Machine has this concept baked in, but I don’t think most people are aware of it, as it’s not promoted as such and based on the questions I get from Macworld readers. Apple makes hay (and rightly so) about the ease of plugging in a drive, responding to a prompt that asks if you want to use it for Time Machine backups, and then never having to interact with it again unless you need to restore files.

But macOS also incorporates support for having multiple active backup volumes used for your same source data at the same time.

1. Plug another drive into your Mac. (See note at the bottom about formatting.)
2. In the Time Machine system preference pane, click Select Disk.
3. In the dialog that appears, select the new drive under Available Disks and click Use Disk.
4. When asked if you want to replace your existing Time Machine volume or use both drives, click Use Both.

(Checking the Encrypt Backups box in step 3 is an excellent idea, too, because it means whenever the volume isn’t mounted, it’s of no use to anyone else without your passphrase to unlock the disk.)

Time Machine begins an initial backup to this volume, which will take as long as the first time you performed a backup with the previous volume. After it’s complete, macOS alternates between the two drives in making backups when both are connected.

But here’s the best part. As soon as the initial backup is finished, you can eject either of the drives, take it somewhere safe away from your home or business, and the Time Machine backup continues on your remaining volume. As frequently as every week or two, swap your offsite volume with the one onsite, and even if you have a fire, theft, or destructive event, you’ll have that nearly up-to-date offsite copy.

If you pair this kind of backup with storing important documents using a sync service, like Dropbox or iCloud Drive, you’ll wind up being able to restore a Mac that experiences a severe crash, or one that’s stolen or you lose on a trip, to nearly the state it was when it became unavailable.

Note on formats: Remember that Time Machine volumes—even in High Sierra and Mojave—cannot be formatted with the new APFS method that Apple requires for SSDs used as the macOS startup volume. Instead, they must use HFS+. You can format a drive as HFS+ in Disk Utility (Applications > Utilities) by erasing it in the Mac OS Extended (Journaled) format. Erasing loses all the data on the disk.

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