macOS Sequoia 15.2 isn't allowing third-party utilities to make bootable backups

The developer of SuperDuper! says what they think is a bug in macOS Sequoia 15.2 has inadvertently broken how the app can create bootable backups — however, it may not be accidental at all.

It's some years since Apple took steps to at least make it harder for people to start up their Macs using external drives. That was a privacy issue, a security issue, and a bit of a pain for anyone who remembers Mac drives failing.

Now according to developer Dave Nanian, Apple has gone further, maybe unintentionally. Nanian is the developer of backup app SuperDuper! and says that since macOS Sequoia 15.2 has changed a feature called the Replicator, his app has been unable to create a bootable backup.

"macOS 15.2 was released a few days ago, with a surprise. A terrible, awful surprise," he wrote in a blog post. "Apple broke the replicator. Towards the end of replicating the Data volume, seemingly when it's about to copy either Preboot or Recovery, it fails with a Resource Busy error."

What this means is that at the last moment, a backup that is meant to create a bootable copy of a user's macOS system, will fail.

"Since Apple took away the ability for 3rd parties (eg, us) to copy the OS, and took on the responsibility themselves, it's been up to them to ensure this functionality continues to work," continues Nanian. "And in that, they've failed in macOs 15.2."

"Because this is their code, and we're forced to rely on it to copy the OS, OS copying will not work until they fix it," he says.

The claimed bug is specifically to do with creating a backup that a user can then start up their Mac from. It doesn't affect backing up data to external drives.

Consequently, Time Machine is so far reported to be working correctly. There have as yet been no reports of the issue affecting other backup apps.

That may be because other backup apps have stepped away from offering full, bootable backups. Carbon Copy Cloner, for instance, says it can only make a "best effort" to create a bootable backup for specific use cases like migrating between Intel Macs.

Otherwise, the company explicitly says it does not support making a bootable drive as part of a regular backup strategy.

As recently as 2021, Carbon Copy Cloner was making bootable backups, but its founder said those days were limited because of the direction Apple was taking macOS. Apple Silicon uses a Signed System Volume and if that is damaged, reportedly a Mac won't start up — perhaps even if the user had an external boot drive.

"We can't fix it.... Apple has to do so," says Nanian. "It's unfortunate this is happening during this period, since Apple rarely releases updates between now and New Year's."

Maybe Nanian is right this macOS Sequoia 15.2 issue is a bug. But it's at least as likely that it is an intentional final nail in the coffin of third party developers being able to create bootable startup disks.