Copying Apple's system is now an Apple-proprietary endeavor; we can only offer "best effort" support for making an external bootable device on macOS Big Sur. We also do not generally recommend that users attempt to make their backups bootable — 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.

In the past, a bootable backup was an indispensable troubleshooting device that even novice users could rely upon in case their production startup disk failed. The reliability of Apple's External Boot solution has waned in the past several years, however, and the situation has grown starkly worse on the new Apple Silicon platform. Apple Silicon Macs will not start up (at all) if the internal storage is damaged or otherwise incapacitated, so there is very little value, if any, to maintaining a bootable rescue device for those Macs.

It has also grown increasingly difficult to make a copy of the operating system. Starting in macOS Big Sur (11.0), the system resides on a cryptographically sealed "Signed System Volume" that can only be copied by an Apple-proprietary utility. That utility is very one-dimensional; choosing to copy the system requires that we sacrifice other backup features, e.g. we cannot copy the system and retain versioned backups of your data. Due to these changes and the limitations of Apple's new "Apple Silicon" platform, creating an external bootable device is not only less approachable for novice users, it's also less likely to serve as a reliable troubleshooting device.

CCC 'Standard Backups' do not include the operating system

By default, CCC does not attempt to make Big Sur (and later OSes) backups bootable. When you configure a backup of your startup disk, CCC will back up the contents of the Data volume. That's all of your data, all of your applications, and all of your system settings – everything about your Mac that is customized. You don't have to be able to boot your Mac from the CCC backup to restore data from it. You can restore individual files and folders using Finder or CCC while booted from your production volume. If you ever need to restore everything from a non-bootable backup, you can install macOS via Recovery mode (e.g. onto a replacement disk), then migrate data from the backup via Migration Assistant. CCC backups are compatible with Migration Assistant, and we support that configuration.

You can install macOS onto your backup to make it bootable

If you have an existing Standard Backup on a non-encrypted APFS volume that you would like to make bootable, you can install macOS onto that volume:

  1. Download and open the macOS Installer: [Catalina] [Big Sur]
  2. When prompted to select a disk, click the Show All Disks... button and select your backup disk
  3. Proceed to install macOS onto your backup disk

Configuring a backup with the Legacy Bootable Backup Assistant

If you would like to configure CCC to establish a bootable backup of your Mac running Big Sur, you can use the Legacy Bootable Backup Assistant. After selecting your source and destination volumes, click on the Destination selector and choose Legacy Bootable Backup Assistant...

Click on the destination selector to access the Legacy Bootable Backup Assistant

Catalina users: The Legacy Bootable Backup Assistant is not applicable. On Catalina, CCC will use its own file copier to make a backup of both the System and Data volumes.

On Big Sur, CCC will use Apple's APFS replicator (named "ASR") to create the initial backup. For subsequent backups, CCC will use its own file copier to keep your Mac's Data volume backup up to date. CCC will not update the System on the destination.

The Legacy Bootable Backup Assistant 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 copy 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).

Standard Backup

This option is the default behavior when not using the Legacy Bootable Backup Assistant. CCC presents this only as a reminder that non-bootable options are available, and sometimes more palatable, e.g. if you have an existing encrypted backup volume that you would like to continue to use without erasing or decrypting it.

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.

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 "Standard 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 backup 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 Standard 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 back up 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 backup volume. To enable FileVault on the destination, you can boot from the backup volume and enable FileVault in the Security & Privacy Preference Pane.

Related documentation