Another warning: Don't convert your Time Machine volume from HFS+ to APFS

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Months after the release of macOS 10.13 High Sierra, folks are still having problems with limitations of the new Apple File System (APFS) format required for SSDs that run High Sierra, and which you can optionally upgrade other drives to use. That includes your columnist, who biffed a Time Machine question that’s now updated for accuracy.

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.

The archive becomes useless, because APFS doesn’t support hard links. These are a special kind of alias. A soft link is a pointer to a destination file that looks to the operating system like a pointer. A hard link looks to the operating system like an actual file, even though it’s just a pointer. This allows a single copy of a file to be in a filesystem, but have many pointers that reference it, and they can be manipulated and copied as if they exist in multiple places.

Time Machine backups start with a full backup of a drive for every file, and then in subsequent backups it creates folder-based snapshots that use a mix of hard links for files that haven’t changed and new files for ones that have. This makes Time Machine accessible through the Finder as well as through the Time Machine app’s graphical interface.

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A Time Machine HFS+ volume is rendered effectively useless whe converted to APFS.

Because APFS lacks hard link support, converting an HFS+ volume to APFS destroys those links and replaces them with broken soft-link aliases. Thus, Macworld reader Yousif noted to me on Twitter that he’d upgraded his HFS+ Time Machine volume to APFS, but he couldn’t copy the backups.backupdb folder, because the aliases were broken. He received a “the operation can’t be completed because it isn’t supported” error. I tried this with individual files that existed on the APFS volume and were not aliases, and received the same error.

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Time Machine files, even ones that aren’t aliases, can’t be copied.

It appears that all the individual copies of files that Time Machine made are intact, so you could manually browse folders to find older versions. That’s better than entirely losing those archives, but it’s not fun, and being unable to copy them directly make them near useless. There doesn’t appear to be any way yet (and possibly ever) to copy that folder to another drive or to restore the hard links, though I would think a developer might be able to write a utility that could handle it.

You can reformat an APFS drive back to HFS+, but it requires erasing the drive completely. Time Machine will offer to handle the erasure and formatting if you try to use an APFS drive for Time Machine. But that, of course, doesn’t restore your archives, either.

There’s no advantage to using APFS on hard drives, and the filesystem isn’t ready for (or maybe will never come to?) Fusion drives that pair an SSD and hard drive for affordability, so I reiterate my advice: don’t upgrade drives manually to APFS.

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