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.
Unfortunately, it has come to our attention that some hard drives that are pre-formatted for macOS are stamped with the same UUID at the factory. As a result, this situation can arise if you own and attach two "factory fresh" hard drives to your computer that came from the same manufacturer.
Solution
Reformatting one of the affected volumes will resolve the problem, however there is a non-destructive solution:
- 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
Note: This procedure may cause bootability problems for a volume that is intended to boot non-Apple computers (aka "Hackintoshes"). Those issues are beyond the scope of our support.
Identity problems specific to Western Digital hard drive enclosures
We have been tracking an issue that can lead to CCC producing the alert described above in cases where a duplicate device is not physically present. Occasionally Western Digital volumes will drop offline (especially during a sleep/wake cycle, and sometimes in the middle of a backup task), but the macOS diskarbitration service errantly retains the virtual device object. When the volume remounts, it is assigned a new device identifier and virtual device object. At that point, any application that asks the macOS diskarbitration service for a list of disks and volumes will get duplicate values for the WD device. Most applications wouldn't care about the duplicate devices, but CCC tracks both mounted and non-mounted devices so that CCC can mount the source and destination at the beginning of the task, if necessary.
CCC works around the underlying macOS issue in every case where it's practical. The one case where it is impossible to reliably work around the issue is in cases where the affected volume is not mounted, but is physically attached to your Mac and currently has duplicate virtual objects on record in the diskarbitration service (both not mounted). If you encounter this scenario, please report this problem to us via the Report a Problem menu item in CCC's Help menu so we can add your OS and device details to our open problem report with Apple (rdar://28972958).
If you ever see two mounted instances of your Western Digital device in the Finder, you should immediately unmount the device, detach it from your Mac, and then restart your computer. In most of the cases we've seen, the duplicate instances of the device are unmounted and therefore harmless. In a couple cases, however, macOS mounted two instances of the volume and the volume wound up corrupted.
Potential workaround
Western Digital's Support Knowledgebase states that the Put hard disks to sleep when possible setting should be disabled when using their external USB hard drives. If you're using a Western Digital external USB device, open the Energy Saver Preference Pane in the System Preferences application and uncheck the box next to the Put hard disks to sleep when possible setting.