A data-only backup is a complete backup of all of your data, settings, and applications
In a handful of cases, CCC will recommend or require that you create a data-only backup of a macOS System volume. If you're unfamiliar with the APFS Volume Group concept that Apple introduced in macOS Catalina, you can learn more about it here:
For a data-only backup, CCC copies the entire Data volume within that APFS volume group. The System volume, which contains only about 12GB of read-only system files that are installed by the macOS Installer, will not be copied. A data-only backup is not bootable, however the backup can be made bootable by installing macOS onto it, or you can use the data-only backup as a source to Migration Assistant.
In some cases CCC will configure your task for a data-only backup automatically when the option is presented to you. You can perform the following steps to manually configure a data-only backup.
- If you would ultimately like to make the backup bootable, erase your destination volume as APFS in Disk Utility
- Click the Show Sidebar button in CCC's toolbar
- Drag the Macintosh HD - Data volume from CCC's sidebar onto the Source selector
- Click on the Destination selector and choose your destination
If CCC was unable to create a full system backup, for example because your startup disk has filesystem corruption or is reporting media read/write failures, you can install macOS Catalina onto the backup disk to produce a bootable backup.
- Download the macOS Catalina Installer
- When prompted to select a disk, click the Show All Disks... button and select your backup disk
- Proceed to install macOS Catalina onto your backup disk
Should I then restore the backup?
After installation has completed, you may use CCC to restore your Mac's internal disk from the backup. APFS corruption is often asymptomatic, however, and we do not recommend proceeding with a recovery unless your system is not functioning correctly.
If your Mac is not behaving correctly and you would like to proceed with a recovery, it's important to erase the internal disk before restoring from the backup. In most cases Disk Utility is unwilling or unable to repair APFS filesystem corruption, therefore it is imperative that you erase the disk that had corruption to remove that corruption from the disk.
If the macOS Installer disallows installation of macOS onto your backup disk, you can use Migration Assistant to migrate data from your data-only backup to a clean installation of macOS. For example, if your startup disk is corrupted or had to be replaced, you could follow these steps to reinstall macOS and restore your data:
- Boot your Mac while holding down Command+R to boot into Recovery Mode
- Use Disk Utility to erase your Mac's (new) internal disk as APFS (see this Kbase article for additional guidance)
- Quit Disk Utility
- Select the Reinstall macOS option and proceed to install macOS onto your new disk
- When macOS boots for the first time on your new disk, you will be prompted to migrate data — accept the migration offer
- When prompted to select a source for the migration, select your CCC data-only backup volume
If you have established a valid, bootable backup volume (e.g. by installing macOS onto your data-only backup), you can reconfigure your CCC task to update the System volume on the destination (as necessary). Likewise, if you created a data-only backup task in error, you can reconfigure the task for a normal, bootable backup with these steps:
- Click the X button in the Source selector to remove the current source selection
- Click on the Source selector and choose your Mac's startup disk as the source to the task
CCC will determine if a bootable clone configuration is possible with the current source and destination. If the configuration is accepted, save the task and you're all set. If the configuration requires changes, CCC will present the options to you.