While macOS can mount and interact with these drives, Time Machine cannot.
To judge by a recent spate of email, Mac users are using Windows and Unix/Linux compatible drives more than ever. Many people have asked why their external drive appears and is grayed-out in the Time Machine preference pane’s excluded list. (Open the Time Machine preference pane and click Options to see that list.)
That’s because the drive isn’t formatted in a way that Time Machine can back up files from. While I’ve covered this in previous columns, it’s worth a quick refresher, given how many people are apparently currently dealing with this.
- Time Machine volumes must be HFS+ formatted, listed in Disk Utility as “Mac OS Extended (Journaled)”.
- Time Machine can only back up volumes formatted either as HFS+ or the newer APFS format.
Read more at MacWorld.com