Discovered a bug in the file system in macOS 10.15.5 that can prevent users from creating a clone that is bootable of your drive system. Sure it’s a bug and not a deliberate choice of Apple?

bug macos 10.15.5

The developer Mike Bombich of Carbon Copy Cloner has explained that his team has discovered the problem in the file system Apple (APFS) when he tried to create a bootable backup on a beta version of macOS 10.15.5.

According to Bombich, the bug prevents Carbon Copy Cloner to use your system copy of the file you need to create a backup of the initial launch of a volume on the macOS system Catalina:

The system call chflags () can no longer set the flag SF_FIRMLINK on a folder of a volume APFS. Instead of failing with an error code that we found, it fails silently, but does not set alcunflag special. This is a bug in the implementation of the file system APFS of chflags: if a system call is not doing what you ask them to do, it should return an error code, not a success.

It is not necessary to set many of these flags, or set them frequently, just the first backup of the volume in the macOS system. This operation is essential to the functionality of a volume group, APFS, and then the inability to set these flags means that the new full system backup created on 10.15.5 and later will not be bootable, and will look as good as if none of your data is present on the target (to be clear, however, is backed up all the data). A bit the opposite of what we are trying to do with CCC. It is difficult to find kind words to express my feelings towards Apple at this moment.

I am extremely disappointed by the fact that Apple introduce this type of bug in an update of the operating system.

A positive note is that the existing backups created in macOS 10.15.4 and earlier are not affected, as the bug has no effect on the ability of CCC to retain the data, nor does it affect the integrity of the filesystem on a boot disk or on a backup disk. In short, the impact of this error is limited to the initial creation of a bootable backup on macOS 10.15.5.

In the meantime, users who want to create a new backup of a volume 10.15.5 on a blank disk should replace their copy of the CCC with CCC 5.1.18 beta, then follow these steps to start the app.

  1. Click on the button X in the selection box Destination to delete the destination selection.
  2. Click on the selector and the Destination, and re-select the destination volume.

CCC will also guide users through the steps of creating a bootable backup or a backup Only data. The new feature uses the utility Apple Restore Software (ASR) and is documented here.

Bombich has informed Apple of the bug, but does not exclude that it may be a fix yet wanted by Apple to prevent third parties to create firmlink. If so, according to Bombich it would be “even worse than a bug”, because the system currently records a success when it should report an error, not to mention the fact that the lack of Apple documentation on the change is hostile to third-party developers that rely on documented features.