Time Machine can’t make use of Apple File System (APFS) volumes. This is a significant problem that many Macworld readers want to make sure others know about. This has been known for months, and because you typically have to take steps to convert a drive to APFS, it hasn’t been an issue for most people.

You can accidentally convert a drive to APFS without macOS warning you that you’re about to do so. If you use the Finder option to encrypt a drive in High Sierra, macOS converts the format to APFS before encrypting.

Macworld reader Chee had this happen to them, and wrote in to ask:

Is there a way to undo this process, or at least recover my Time Machine backup going back 2 years and re-setup my external HD?

Yes, although it’s a pain in the patootie, because while converting from HFS+ to APFS and encrypting a drive are all non-destructive and happens in place, converting back to HFS+ requires erasing everything on the disk. Here’s how to make this happen:

  1. Obtain a new disk big enough to hold the existing Time Machine backups.
  2. Format that disk for HFS+.
  3. Follow the instructions in this article to copy your Time Machine backups from the APFS formatted disk to the HFS+ formatted one.
  4. Enable Time Machine on the APFS disk.
  5. macOS will prompt you to erase the disk and reformat it as HFS+. Click Erase.
  6. When the drive is ready, turn off Time Machine, and copy the backups back from the HFS+ disk you used in step 3.

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