Things become even more confusing if you add an M1-based Mac into the mix. Unfortunately, once you do this, you can no longer copy to the backup until you delete the System volume, so it’s best to stick with SuperDuper 3.2.5’s data-only backups. The current version of SuperDuper has other issues with Big Sur, so SuperDuper’s workaround involves downgrading to SuperDuper 3.2.5, using that to make a data-only backup, and then installing Big Sur on the backup drive if you need to boot from it. ChronoSync suggests installing Big Sur on an empty drive first and then using it for your data-only backup. Carbon Copy Cloner can make a one-time bootable duplicate of an Intel-based Mac (but you must boot from it to install macOS updates) and for M1-based Macs recommends installing Big Sur onto a data-only backup after creating it. On the plus side, Apple has said it plans to fix asr, but who knows when, or how completely, that will happen.Īll three of the leading apps for making bootable duplicates have come up with workarounds. In theory, Apple’s asr (Apple Software Restore) tool makes this possible, but it didn’t work at all until just before Big Sur was released, still has problems, and even now cannot make a bootable duplicate of an M1-based Mac boot drive. This change increases security even more, but it also prevents all backup apps from creating bootable duplicates because they cannot sign the backed-up System volume. Thus, Big Sur is actually booting from a cryptographically signed, immutable reference to a cryptographically signed read-only volume.) It first takes the additional step of creating an immutable APFS snapshot-a reference to the volume at a particular point in time-and starts up from that snapshot. (In fact, Big Sur doesn’t even read files directly from this System volume to boot your Mac. With Big Sur, however, Apple went a step further, adding strong cryptographic protections when storing system content on what is now called a Signed System Volume. Eventually, all the leading apps figured out how to do this: see “ Carbon Copy Cloner 5.1.10” (26 August 2019), “ ChronoSync 4.9.5 and ChronoAgent 1.9.3” (11 October 2019), and “ SuperDuper 3.3” (30 November 2019). Now a bootable duplicate had to have a System and a Data volume, and they had to be combined correctly into an APFS volume group. This architectural change forced backup apps that make bootable duplicates to jump through hoops, since they couldn’t just read and write data anymore. The System volume is also read-only, so malicious software cannot modify the operating system, whereas the Data volume that contains your files remains read-write so you can install apps and create and modify documents. The two volumes appear as a single entity in the Finder and wherever you might select or navigate files. A System volume holds all the files macOS needs to operate, while the Data volume contains only your data. In 10.15 Catalina, Apple introduced APFS volume groups, a way of bundling separate volumes together to create a bootable macOS. Why Bootable Duplicates Have Become Difficult to Make And once we’ve done that, we can reassess the role of a bootable duplicate in a modern backup strategy. To understand why this seemingly simple task-just read all the data from one drive and write it to another-is causing such consternation, we need to step back briefly. That’s the complexity of creating a bootable duplicate of your startup volume, also known as a clone. However, there is one general concern that has caused us to hesitate to recommend upgrading. Is it time to upgrade to macOS 11 Big Sur? I’ll write more about that soon. The Role of Bootable Duplicates in a Modern Backup Strategy
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