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.
