CCC makes non-proprietary backups – when you configure a source and a destination to a CCC task, the objective is to have the same files on the source and destination, and in the same hierarchical arrangement. To achieve that objective, CCC will make exact copies of your source files on the destination, and CCC will also remove content from the destination that is not on the source. The removal of content from the destination is important to consider when you select a destination volume. If you already have content on the destination that's unrelated to the source, that content may be deleted.
Dedicate a volume to the backup task
We recommend that you dedicate a backup volume to each backup task. If you want to store other data on the backup disk (i.e. content that is unrelated to the source that you're backing up), store it on other volumes that are not specified as a CCC destination. You can add volumes to your backup disk in Disk Utility.