Watch out! macOS High Sierra could format your Time Machine drive using APFS, and that's not good

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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?

No, unfortunately, unless you have been rotating drives or have a backup of your Time Machine volume stored elsewhere.

While converting from HFS+ to APFS and encrypting a drive are all non-destructive and happens in place, APFS doesn’t support the hard links—multiple pointers on a volume to single instances of a file—used by Time Machine. Your files are still there, but not in Time Machine accessible archives. The Time Machine archive, backups.backupdb, can’t be copied because of broken aliases, and the archive can’t be restored because the hard links are broken. You have to navigate through the archive folder manually to find files you want to retrieve old versions of.

And if you convert the drive back to HFS+, you have to erase everything on the disk. You should keep the files from your APFS-formatted volume, however, as you can recover old versions with a lot of navigation from them. Unfortunately, you may need to obtain a new drive and format it for HFS+ so you can still access older versions of files on your converted APFS volume and make Time Machine backups with a new drive.

Correction: This article originally stated you could copy the Time Machine archive successfully back from an APFS volume to an HFS+ volume. That is unfortunately not possible.

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