If you use Time Machine, you know that macOS asks you whenever you mount a new external drive whether or not you want to use it as another Time Machine destination. One of the backup service’s best features is that it can create archives on multiple volumes at once, and remembers backup volumes when they’re removed, so you can rotate through disks and keep one or more offsite.
However, you might mount a drive and want to use it with Time Machine and be unable to select it. That’s typically because the volume isn’t formatted using the old standby, HFS+, labeled “Mac OS Extended (Journaled)” in Disk Utility.
Time Machine can only create backup archives on HFS+ volumes because of some of the peculiar properties of how it creates snapshots by referencing files with hard links. These links allow the same file to appear as if it’s in multiple places on a single volume, even though only a single copy of it occupies space on the disk.
Despite Apple’s adoption of APFS, Time Machine cannot use volumes from drives in this format. It’s more likely that you have a disk formatted at some point for a Windows or Unix system, likely using MS-DOS FAT or ExFAT formats.
If you want to use a disk in one of those formats, first back up any data you need to preserve from the disk: there’s no way to reformat without erasing the drive’s contents. Next, follow Apple’s instructions to erase and reformat it as a Mac OS Extended volume.
When the volume mounts again, Time Machine will recognize it as a destination.
This Mac 911 article is in response to a question submitted by Macworld reader Matthew.
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Author: Glenn Fleishman, Senior Contributor
Glenn Fleishman’s most recent books include Take Control of iOS and iPadOS Privacy and Security, Take Control of Calendar and Reminders, and Take Control of Securing Your Mac. In his spare time, he writes about printing and type history. He’s a senior contributor to Macworld, where he writes Mac 911.