Don't convert your Time Machine volume from HFS+ to APFS
Time Machine can work with APFS volumes, but the shape looks like this:
- Time Machine can archive files from both HFS+ and APFS volumes.
- Time Machine volumes must be HFS+.
- You can use Disk Utility to upgrade a Time Machine HFS+ volume to APFS without a warning. You’d think Disk Utility would detect the Time Machine backup and stop you, but it doesn’t.
- Once upgraded to APFS, the Time Machine backup archive is mostly useless, even though files aren’t destroyed.