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 many Iomega and Western Digital 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" Iomega hard drives to your computer.
Reformatting one of the affected volumes will resolve the problem, however there is a non-destructive solution:
- Quit CCC if it is open
- Download this Reset Volume UUID utility
- Unarchive the Reset Volume UUID utility and open it
- When prompted, select one of the volumes that was identified as having a non-unique unique identifier
- Open CCC and try configuring your backup task again
We have been tracking an emerging Sierra-specific issue that can lead to CCC producing this alert in cases where a duplicate device is not physically present. Occasionally when you unmount and physically detach a Western Digital enclosure from a Mac running macOS Sierra, the macOS diskarbitration service errantly retains the virtual device object. When you subsequently reattach the device, 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 will get duplicate values for the WD device. Most applications wouldn't care about the duplicate devices, but CCC tracks these devices and uses them to mount the source and destination at the beginning of the task, if necessary. Please note that CCC isn't causing this condition, it's just complaining about it.
CCC works around the underlying macOS bug in every case where it's practical. The one case where it is impossible to reliably work around the bug 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. 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.