CCC attempts to automatically manage free space on your destination by deleting older backups as the disk starts to get full. There are a few scenarios that can lead to CCC being unable to fit your backups onto the destination. Here are some things you can do to get past a "destination is full" error.
Use an APFS formatted destination
APFS offers several space-savings features that are not supported by other filesystems. If your source is an APFS volume and the destination is formatted with some other filesystem, some files may consume more space on the destination. If you're backing up to a locally-attached device, click on CCC's Destination selector and choose Backup Volume Setup Assistant to get the destination volume formatted correctly. If you cannot change the destination format, consider the additional suggestions below.
SafetyNet is not part of the backup strategy, rather it is a legacy safety mechanism that can lead to overly-conservative retention of content on your destination volume. When your destination volume is dedicated to the backup task, SafetyNet should be disabled.
- Select your task in CCC's main application window.
- Click on the Destination selector and choose SafetyNet Off from the SafetyNet sub menu.
- If prompted, choose the Remove SafetyNet option to have the existing SafetyNet folder deleted from the destination immediately.
- Click Advanced Settings at the bottom of the window, then select the File Copying Settings tab. Verify that the box next to Protect root-level items on the destination is unchecked.
- If applicable, empty the Trash in the Finder.
- Save and run the backup task.
When you disable the SafetyNet, all files on the destination are subject to permanent removal – any file that does not exist on the source will be deleted. If you have any doubt about whether content on the destination may be removed, click the Preview button in CCC's toolbar to preview the changes before running the task.
Use a larger destination
Ideally, your destination volume will have about twice the capacity as space consumed on the source. That allows ample room for data growth and snapshot retention. If your source data set is larger than, or very close to the capacity of the destination, then it may not be possible to perform safe updates to files on the destination.