SuperDuper! app icon amidst hard drives and a Mac keyboard. Mac utility app SuperDuper! will no longer always fail to make a bootable backup because of a bug in macOS Sequoia — but Apple's fix doesn't remove all problems. The newly-released macOS Sequoia 15.3 has corrected a problem that saw the SuperDuper! backup utility fail at particular step in making a bootable copy of a drive. Developer Dave Nanian reported in December 2024 that the then-new macOS 15.2 had changed a feature called the Replicator.
Nanian thought the change might have been accidental, but whatever the cause, it was causing his app — and presumably other backup utilities — to fail. "Towards the end of replicating the Data volume," he wrote at the time, "seemingly when it's about to copy either Preboot or Recovery, it fails with a Resource Busy error.". Now Nanian has posted an update on his Shirt Pocket Watch blog to say that this has been resolved.
"Just a quick post: macOS 15.3 is now out, and with it, a fix for the broken replicator," he writes. "As such, macOS copying will work again with 'Erase, then copy' backups [and] no update to SuperDuper is necessary.". As Nanian notes, however, this "does not mean [that] boot from the [backup] copy will work in all situations." It solely means that this specific failure has been fixed in macOS 15.3, which suggests it was an error.
Also suggesting that it was a bug is how users in the AppleInsider forums reported that SuperDuper! was sometimes working correctly, even under macOS 15.2. At the same time, forum users also said that Apple's own Time Machine was proving problematic, although there appears to be no connection to the Replicator bug. In fact, SuperDuper! was seemingly the only backup app affected by the bug, but that appears to be because most utilities no longer try to make bootable backups. Apple's Time Machine does not, for example, and Carbon Copy Cloner's developer explicitly says that it can no longer make full bootable backups.
The reason for being unsure whether Apple's original change was intentional or a chance bug, is that Apple has been working for years to make it harder for people to start up their Macs from external drives. That was for entirely laudable privacy and security benefits to the user, but it removed a safety net that long-standing Mac owners were used to relying on. It used to be that having at least one bootable backup was highly recommended, because it meant a failing Mac could be quickly started up from one. But that was back when Macs ran on Intel processors — and before that PowerPC — whereas such backups can't help with Apple Silicon Macs.
That's because Apple Silicon uses a Signed System Volume. If that is damaged, a Mac will reportedly not start up at all — quite possible even if a user does have an external boot drive. William Gallagher has 30 years of experience between the BBC and AppleInsider discussing Apple technology. Outside of AppleInsider, he's best known for writing Doctor Who radio dramas for BBC/Big Finish, and is the De...
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