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. On modern macOS and the Apple Silicon platform, External Boot brings more technical complexity to the table, and requires some compromises to a backup strategy that make it less desirable for the average user.
Additionally, copying Apple's system is now an Apple-proprietary endeavor; we support the mechanics of creating a bootable copy of the system, but our ability to troubleshoot bootability issues is limited. For the average user's default backup strategy, we recommend proceeding with a "Standard Backup". 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.
There are a handful of scenarios where it's handy to have a bootable copy of the system:
- You're migrating to a replacement storage device in an older Intel Mac
- You want to test a new OS version without affecting your production startup disk
- You have extreme uptime requirements, e.g. if anything is wrong with your Mac, you must get it running again within minutes, not hours
When you select a destination to your backup task, CCC's Backup Volume Setup Assistant will offer an option to create a bootable copy of the source.
Making a copy of the startup disk with Apple Software Restore
- Choose your startup disk as the source.
- Select a destination volume on a USB or Thunderbolt device
- In the Backup Volume Setup Assistant, choose the option to make a Legacy bootable clone using Apple Software Restore.
- Click the Allow CCC to erase '(destination)' button.
- Start the task.
- When the task has completed, open System Settings > General > Startup Disk and set the destination volume as the startup disk to prompt macOS to authorize users on that volume. You may reset the startup disk selection to your production startup disk after authorizing users on the destination volume.
The Backup Volume Setup Assistant is presented automatically when you select a destination volume. To present it again for a volume that is already selected, click on the Destination selector and choose Backup Volume Setup Assistant….
Things to consider before relying on an external macOS boot device
Supportability
The bootable-cloning procedure relies on Apple's proprietary APFS replication utility, which is outside of our developmental control. We welcome feedback on this functionality, but we may not be able to offer in-depth troubleshooting assistance for the bootability of the destination device beyond the suggestions offered in our External Boot Troubleshooting kbase article. Don't let that stop you from asking for help; we'll make a valiant effort to get your backups working however you choose to approach them.
Reliability when the internal storage fails
Solid state media often fails to a read-only condition, which may still support External Boot. An external bootable device will not serve as a rescue disk, however, if an Apple Silicon Mac's internal storage fails completely.
Third-party kernel-space drivers aren't loaded during External Boot
Software that depends on kernel-space drivers may not work when an Apple Silicon Mac is booted from external storage. Third-party kernel extensions and kernel-space drivers will only be loaded when an Apple Silicon Mac is booted from its internal storage.
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 copy of the system?
Yes. The macOS system volume can only be copied by Apple's ASR utility, and that utility only works by erasing the destination. You must dedicate a volume to your bootable copy of the system.
Can I use the destination device for storing other data as well?
On a separate, dedicated volume, yes. We recommend that you add an APFS volume to the destination APFS container and use that new volume for your other content. As long as the system copy 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 destination 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 the copy of the system.
Related documentation
If I continue to make regular backups to the destination, will it remain bootable?
If you keep the task configured to copy the source via Apple Software Restore and run the task regularly (i.e. via manual or scheduled backups), then the copy should remain bootable. If you change the configuration of the task via the Backup Volume Setup Assistant to a "Standard Backup", then the destination may not remain bootable.
Can I exclude some content when making a bootable copy of the system?
No. The macOS system volume can only be copied by Apple's ASR utility, and that utility can only make a complete, byte-for-byte exact copy of the startup disk. If you must exclude content from the initial copy, then we recommend that you proceed with a Standard Backup.
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 "Allow CCC to erase {destination}" option. Other volumes on the same physical device will be unaffected. Regardless, we never recommend that you target 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:
- Click Volumes in CCC's sidebar
- Select the volume you would like to delete in CCC's sidebar
- Right-click on the selected volume and choose the option to delete that volume
Can I make the system copy on 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 destination volume. To enable FileVault on the destination, you can boot from the system copy and enable FileVault in System Settings > Privacy & Security > FileVault.