The makers of Carbon Copy Cloner (CCC) have discovered a serious issue in the latest version of macOS that affects the creation of bootable backups.

According to CCC’s Mike Bombich, Apple changed something in the chflags() system call. As a result, when trying to initialize a bootable backup, it exits with a success exit status, but actually fails. Unfortunately, it does not generate an error code when it fails, as it should.

Bombich makes it clear this issue has no impact on existing backup drives or the startup drive, but only the process of creating a new bootable drive. Fortunately, CCC has a plan in place.

“Last year at Apple’s Developer Conference, Apple suggested that backup software should use Apple’s ‘Apple Software Restore’ (ASR) for cloning APFS volume groups,” writes Bombich. “Initially I dismissed this – I shouldn’t have to use Apple’s black-box utility to do my job, I prefer to take full responsibility for my backups. Anticipating a world in which Apple continues to restrict access to APFS rather than grant it, though, we decided to invest a fair amount of time evaluating this functionality, and we’ve been beta testing it for the last 8 months. I don’t like to lean on ASR for general backups because it has some shortcomings and doesn’t give any insight into its internal activity (e.g. files copied, errors encountered), but in this very narrowly-defined case, we can leverage Apple’s proprietary utility just to establish bootable backups. We posted a beta last Sunday with new UI around this functionality, and we intend to continue producing bootable backups by leveraging ASR for the initial backup.”

As Bombich points out, there is no way of knowing if this change was intentional, in an effort on Apple’s part to force developers to use ASR and not use firmlinks. If so, at least CCC is prepared. If not, a fix should be forthcoming.