Occasionally a circumstance arises in which CCC presents the following error message before creating or running a backup task:
CCC found multiple volumes with the same Universally Unique Identifier that was associated with the volume you designated as the source/destination for this task.
CCC cannot proceed with confidence in having correctly identified the volume you originally chose when you configured this backup task. Unmount one of the conflicting volumes and try the task again, or please choose "Ask a question" from CCC's Help menu to get help resolving the issue.
Most modern operating systems apply a universally unique identifier to a new volume when you format that volume (e.g. in Disk Utility). Volumes should never have the same identifier, these identifiers are called "universally unique" because they're supposed to be unique, universally! Wikipedia notes that, for 122 bit UUIDs, there is a 50/50 chance of having a single duplicate UUID if 600 million UUIDs were allocated to every person on Earth. The chances of two volumes having the same UUID should, then, be slim enough that the UUID can be reliably used to positively identify the source and destination volumes.
Given these odds, it is statistically more likely that CCC's discovery of a duplicate UUID is due to a hardware or software problem rather than to two volumes randomly having the same UUID. Therefore, CCC makes the conservative decision to not back up to either volume if another volume with the same UUID is detected.
Solution
Reformatting one of the affected volumes will resolve the problem, however there is a non-destructive solution:
- Click Volumes in CCC's sidebar
- Hold down Control+Option and click on one of the volumes that was identified as having a non-unique unique identifier in CCC's sidebar
- Choose the "Reset UUID" contextual menu item
- Try configuring your backup task again