Time Machine’s main purpose is to back up an internal startup volume, but it can also archive the contents of other volumes on an internal drive and volumes on externally attached drives. Check the Time Machine preference pane to make sure those other volumes aren't excluded if you want them backed up. (But you can't include the Time Machine volume itself!)
However, once Time Machine has started to archive those other volumes, you might wonder how to find their contents to restore older versions or the entire set of files. Apple doesn't provide the same ease of navigation backwards nor full restore with volumes other than the one set to start up the Mac, but the data is all there.
You can access external volumes through any of the following means:
In the Finder, open a window from the external volume and then select Enter Time Machine from the Time Machine system menu. This should take you to a view that lets you browse the contents of that volume.
Select Enter Time Machine and then use the sidebar to select a volume.
Backups.backupdb folderon the Time Machine volume (connected directly or as a network shared volume) and open it. Inside, you'll find each computer backed up by name. Open the correct name and then open the Latest folder. All volumes appear there.
These external volumes aren't readily available when using Migration Assistant or recovering a computer via Disk Utility's restore operation in macOS Recovery, but can be copied later from the Time Machine backup after you've set the Mac up again with its startup volume.
This Mac 911 article is in response to a question submitted by Macworld reader Fred.
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