The creator of Carbon Copy Cloner says an Apple File System bug in macOS Catalina 10.15.5 can prevent users from making a bootable clone of their system drives.
In a blog post on Wednesday, software developer Mike Bombich said the CCC development team had uncovered the issue in the Apple File System (APFS), when attempting to create a bootable backup in a beta version of macOS 10.15.5.
Bombich says the bug prevents Carbon Copy Cloner form using its own file copier to create an initial bootable backup of a Catalina System volume.
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.
Suffice it to say, though, I’m extremely disappointed that Apple would introduce this kind of bug in a dot-release OS update. We’ve seen 5 major updates to Catalina now, we should expect to see higher quality than this from an operating system.
Bombich says existing backups created in macOS 10.15.4 and earlier are unaffected, the bug has no effect on CCC’s ability to preserve data, and it does not affect the integrity of the filesystems on a startup disk or a backup disk.
Users that wish to create a new backup of a 10.15.5 volume to an empty disk should download and install the CCC 5.1.18 beta, then follow the steps listed below after launching the app.
- Click the X button in the Destination selector box to clear the destination selection.
- Click on the Destination selector and reselect the destination volume.
The app will then guide users through creating a bootable backup or a data-only backup. The new version of CCC uses Apple’s Software Restore (ASR) utility and is documented here.