Cloning macOS System volumes with Apple Software Restore

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Apple introduced a flaw in macOS 10.15.5 that prevents non-Apple applications from creating a special link between the System and Data volumes in an APFS volume group. In layman's terms, that means that we cannot use our own file copier to create an initial bootable backup of your Mac's startup disk on macOS 10.15.5. When you configure a CCC backup task to make a clone of a Catalina 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 between your source and destination.

What to expect as you configure your first backup task

When you select an APFS volume group (e.g. your macOS Catalina startup disk) as the source to a task, and a destination that is not an already-established APFS volume group, 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 accommodate 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. 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 don't 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.

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 that vestigial volume 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 10.15.5, yes. As of macOS 10.15.5, we're required to use Apple's APFS replicator to establish a 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 in the future.

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 10.15.5, 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.

Cloning macOS to a disk image

Regretfully, we're unable to offer support for cloning macOS Catalina system files to a disk image file (i.e. a sparseimage or sparsebundle disk image file). During our testing of ASR, we discovered a troubling interaction between ASR and disk image volumes when the disk image file resides on a volume that is nearly full (FB7348548). The combination of ASR and disk images does not meet our standards for providing a reliable and robust backup.

CCC will allow "Data Only" backups to disk images. Rest assured that all of your data and applications remain backed up, and you would be able to migrate the contents of that backup to a fresh installation of macOS via Migration Assistant. The only change is that the macOS system files will not be copied to a destination disk image.

Related documentation