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. CCC 6 will not attempt to create a bootable backup of Big Sur by default, however the functionality is available via the Legacy Bootable Backup Assistant.
How are bootable copies made differently on macOS Big Sur?
When configured via the Legacy Bootable Copy Assistant, CCC will use Apple's APFS replication utility, "ASR", to establish a bootable copy of your startup disk. Apple's 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 legacy bootable copy 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 backup
- Add a new, dedicated backup volume to an existing APFS destination (if there is enough free space)
- Proceed with a Standard 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 Backup" see Creating legacy bootable copies of macOS (Big Sur and later).
Does my CCC backup have to be bootable for me to restore data from it?
No, in fact we no longer recommend that you attempt to make your backup bootable. 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).
Related resources
After CCC has established an initial bootable copy, will it keep the destination System volume up to date?
No. We would like to offer this functionality, but doing so involves some unacceptable compromises. 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 every time an OS update is applied to the source. Doing so would remove all snapshots on the destination, and would take quite a bit longer than an ordinary incremental 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.
Related resources
- Creating legacy bootable backups of macOS Big Sur
- How to restore from your backup
- Using Migration Assistant to restore your startup disk from a CCC backup
- Frequently asked questions about CCC and macOS Catalina (many of these are also applicable to Big Sur)
- Best practices for updating your Mac's OS
- macOS Big Sur Known Issues
