Starting in macOS Big Sur (11.0), the system 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 cannot use its own file copier to establish an initial bootable backup of your Mac's startup disk. When you configure a CCC backup task to make a clone of a Big Sur startup volume, CCC will use Apple's APFS replicator (named "ASR") to create the initial clone. For subsequent backups, CCC will use its own file copier and will copy just the differences from your Mac's Data volume.
What to expect as you configure your first backup task
When you select an APFS volume group (e.g. your macOS Big Sur startup disk) as the source to a task, and a destination that is not an already-established Big Sur volume, CCC will offer some choices for how to proceed with the task depending on how the selected destination is configured. We recommend that you dedicate a volume to the backup task, because the volume will have to be erased to establish a bootable backup.
Erase the destination
When you select this option, CCC will configure the task to use Apple's APFS replicator to clone the selected source to the selected destination. When you start the task, the destination will be immediately erased. SafetyNet is not applicable in this configuration, so be sure that you have selected an empty volume, or a volume that has data that may be deleted (e.g. an old backup).
Add a volume
If the selected destination is an APFS-formatted volume, and if the volume's container has enough capacity to accommodate a complete backup of the source volume, CCC will offer an option to add a volume to the destination APFS container. When you choose this option, your selected destination is not erased, nor affected at all by the backup task. Instead, CCC will add a new volume on the destination and use the new volume as the dedicated backup for the source.
Data Only backup
If you cannot or do not want to make a bootable backup of the selected source, and you also do not want to erase the selected destination, you can select the Data Only option. When you select this option, CCC will select the Data volume of your selected source as the source to your task, and will not erase the selected destination. This option is convenient if you have an existing encrypted backup volume that you would like to continue to use without erasing or decrypting it. Data-only backups will not be bootable, however you can use these backups as a source to Migration Assistant, e.g. to migrate data to a fresh installation of macOS.
Things you should know before relying on an external macOS Big Sur boot device
This procedure relies on Apple's proprietary APFS replication utility, which is outside of our developmental control. We welcome feedback on this functionality, but we cannot offer in-depth troubleshooting assistance for problems that Apple's replication utility encounters.
- Whether the destination is bootable depends on the compatibility of your Mac, macOS, and the destination device. We cannot offer any troubleshooting assistance for the bootability of the destination device beyond the suggestions offered in our External Boot Troubleshooting kbase article.
- The destination may not remain bootable if you subsequently update the data on the backup after applying OS updates to the source.
- Apple's replication utility may cause a kernel panic while cloning to the internal storage on Apple Silicon Macs, which could prevent you from restoring your Mac's system from the backup.
- Apple Silicon Macs will not boot at all if the internal storage fails. An external bootable device will not serve as a rescue disk for that scenario.
For the reasons noted above, we do not generally recommend that you attempt to make your backups bootable; we recommend that you proceed with a "Data Only" backup instead. You can restore all of your documents, compatible applications, and settings from a standard CCC backup without the extra effort involved in establishing and maintaining a bootable device.
Frequently Asked Questions
When the task started running, the destination was renamed to the same as the source. And what's this "ASRDataVolume" volume?
Highly perceptive people may notice that the name of the destination volume changes as Apple's volume replicator goes to work. An additional volume may appear in CCC's sidebar as well. This is normal. These volumes will be aggregated into a "volume group" and presented as a single volume, and CCC will rename the destination to its original name when the replication is complete.
If Apple's APFS replication utility fails and you see an ASRDataVolume or ASRNewVolume persisting, then you may delete those vestigial volumes in Disk Utility. Simply select the volume, then click the "-" button in the toolbar.
Do I have to erase the destination to make a bootable backup?
If your Mac is running Big Sur, yes. As of macOS Big Sur, we're required to use Apple's APFS replicator to establish a bootable clone of an APFS volume group. We're unable to leverage the SafetyNet feature, and it's no longer appropriate to store other data on the backup volume. You must dedicate a volume to your bootable backup. Once your bootable backup is established, CCC will use its own file copier to update the destination Data volume in subsequent backup tasks.
Can I use my backup disk for other purposes as well?
Yes. We recommend that you add an APFS volume to the destination APFS container and use that new volume for either your dedicated CCC backup, or for your other content. As long as the CCC backup and the other content are stored on separate volumes, these can coexist peacefully on the same physical device. Likewise, you may add a partition to your backup disk if the destination is not APFS formatted. For example, if you have an external hard drive that already has content on an HFS+ formatted volume, you can add a partition to the disk and use the new partition for your CCC backup.
Can I exclude some content from the initial backup?
If your Mac is running Big Sur, then it is not possible to exclude content and produce a bootable backup. If you must exclude content from the initial backup, then we recommend that you proceed with a data-only backup. If you would like to make that backup bootable afterwards, then you can install macOS onto the backup volume.
Related documentation
I already have other volumes on my backup disk. Will those be erased?
No, only the selected destination volume will be erased when you proceed with the "Erase {destination}" option. Other volumes on the same physical device will be unaffected. Regardless, we never recommend that you back up to a disk that has data on it that is not backed up elsewhere. If those other volumes are not yet backed up, then back up that data before proceeding.
I added a volume, but I don't want the extra volume after all. Can I delete it?
Yes. Choose Disk Utility from CCC's Utilities menu, select the volume you would like to delete, then press the "–" button in the toolbar to delete that volume.
Can I clone to an encrypted volume?
You may select an encrypted volume as the destination, but the volume will be erased, and will not be encrypted when the task completes. Apple's APFS replication utility will not preemptively enable FileVault on the cloned volume. To enable FileVault on the destination, you can boot from the backup volume and enable FileVault in the Security & Privacy Preference Pane.