Bug In macOS Catalina 10.15.5 Prevent User Creating A Bootable Backup

Backup software Carbon Copy Cloner developer Mike Bombich said in a blog post that there is an APFS bug in macOS Catalina 10.15.5, which will prevent users from successfully creating a bootable backup of the system disk.

Bombich detailed this specific circumstance in two paragraphs:
The chflags() system call can no longer set the SF_FIRMLINK flag on a folder on an APFS volume. Rather than fail with an error code that we would have detected, it fails silently – it exits with a success exit status, but silently fails to set the special flag. That's a bug in the APFS filesystem implementation of chflags – if a system call doesn't do what you ask it to do, it's supposed to return an error code, not success.

We don't need to set many of these flags, nor set them frequently – just on the first backup of the macOS system volume. It happens to be essential to the functionality of an APFS volume group, though, so the failure to set these flags means that new full-system backups created on 10.15.5 and later won't be bootable, and it will appear as if none of your data is on the destination (to be clear, though, all of the data is backed up). Kind of the opposite of what we're trying to do here. It's hard to find kind words to express my feelings towards Apple right now.
Backups created with macOS 10.15.4 or earlier are still available, and Mike Bombich says that this is not an issue for most current CCC users, as their existing bootable backups will continue to work and can proceed to be updated.

For anyone who is running 10.15.5 and is backing up a Catalina system volume to an empty disk, Bombich recommends you replace your copy of CCC with the CCC 5.1.18 beta. Open the app:
  • Click the X button in the Destination selector box to clear the destination selection
  • Click on the Destination selector and reselect your destination volume
CCC will then guide you through the procedure of creating a bootable backup

Mike Bombich has notified Apple, so a fix should be coming soon.

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