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With the announcement of macOS Big Sur, Apple has retired Mac OS X (10) and replaced it with macOS 11. As the numeric change would suggest, this is the biggest change to macOS since Apple introduced Mac OS X roughly 20 years ago. The system now resides on a cryptographically sealed "Signed System Volume". That seal can only be applied by Apple; ordinary copies of the System volume are non-bootable without Apple's seal. To create a functional copy of the macOS 11 System volume, we have to use an Apple tool to copy the system, or install macOS onto the backup.

How are bootable backups different on macOS Big Sur?

CCC will use Apple's APFS replication utility, "ASR", to establish an initial bootable clone of your startup disk. This utility does not offer as much flexibility as you've grown accustomed to with CCC on older OSes, in particular it requires that the destination is erased and that everything is copied from the source to the destination. When you configure a new backup of your startup disk on Big Sur, CCC will offer a few options, depending on the size and current format of your destination device:

  • Allow CCC to erase the destination to make a bootable clone
  • Add a new, dedicated backup volume to an existing APFS destination (if there is enough free space)
  • Proceed with a Data Volume backup (this is a complete backup of all of your data, applications, and system settings)

To learn more about these options, and what to expect when running your first "Full Volume Clone" see Cloning macOS System volumes with Apple Software Restore.

Does my CCC backup have to be bootable for me to restore data from it?

No. Bootability is a convenience that allows you to continue working if your startup disk fails, but it is not required for restoring data from a CCC backup. You can restore individual folders and older versions of files (i.e. from snapshots) using CCC while booted from your production startup disk. CCC backups are also compatible with Migration Assistant, so you can use Migration Assistant to restore all of your data to a clean installation of macOS (e.g. on a replacement disk).

After CCC has established an initial bootable backup, will it keep the destination System volume up to date?

No. We would like to offer this functionality, but doing so involves a tradeoff that we think most users would find unacceptable. Due to an inflexibility in Apple's APFS replication utility (ASR), we can only update the destination System volume by cloning both the System and Data volumes together with ASR, and that involves erasing the destination. Doing so would remove all snapshots on the destination, and would take quite a bit longer than an ordinary incremental backup.

Fortunately, updating the System volume on the destination is not something that you have to do frequently, nor even proactively, it's something that you can do if and when required. Simply boot your Mac from the backup volume and apply any updates via the Software Update preference pane in the System Preferences application. This is something that you could even defer until the need arises to restore from the backup.

How do I upgrade my Catalina (or older) backup to Big Sur?

After you upgrade your Mac to Big Sur, and only after you have decided to commit to the Big Sur OS, you may resume the backup of your startup disk to your CCC backup volume. Open CCC and review each of your backup tasks to see if any adjustments are required for the first backup on the new OS.

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