Time Machine offers a powerful continuous archiving tool for files on your Mac. By design, Apple keeps it simple: it lets you select just a single set of volumes (and excluded volumes, folders, and files) on your Mac for all Time Machine backups, Time Machine can back up automatically only in one of three intervals (hourly, daily, or weekly), and whenever Time Machine volumes are plugged in, macOS rotates among them across the specified interval. (For instance, if you have Time Machine Volume A and Volume B and are set to hourly backups, macOS backs up to Volume A at 3 p.m., Volume B at 4 p.m., Volume A at 5 p.m., and so on.)
However, you can gain some control by mounting and unmounting volumes. For instance, say you have a 1TB internal drive and a 4TB external drive. You only mount the external drive from time to time. For backups, you have a 1TB SSD and a 6TB hard disk drive (HDD). You can’t back up 5GB to a 1TB drive.
The solution involves a little dance whenever you want to back up the 4TB drive.
To start with, set up your Time Machine backup to include both your internal drive and your 4TB external photos/data drive and eject the 4TB drive. You’ll have to make sure Time Machine isn’t in the middle of a backup, and you may have to disable Time Machine momentarily to eject it without error. In Monterey or earlier, go to System Preferences > Time Machine; in Ventura or later, go to System Settings > General > Time Machine.
Now, let normal Time Machine backups occur on your 1TB Time Machine drive. Whenever you want to back up the 4TB drive, instead of simply plugging it in, first eject the 1TB Time Machine volume.
Then mount the 4TB data drive and the 6TB Time Machine volume. Time Machine will back up both the internal volume and the 4TB data drive to the 6TB Time Machine volume. When the backup is complete, you can unmount the 6TB Time Machine volume.
Given the scenario, you could also add a third-party utility: have Time Machine only back up your 1 TB internal drive and use Carbon Copy Cloner or Econ Technologies’ ChronoSync to manage archiving or cloning the 4TB drive to an external volume of the same or larger size.
This Mac 911 article is in response to a question submitted by Macworld reader Mark.
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We also discuss How to manage multiple Time Machine backups and how to restore files from a Time Machine backup.