I encountered this problem yesterday morning when my nightly backup failed due to insufficient space on the destination/cloned drive.
Since my source/boot volume was down to 5% free space, I decided I should free up some space. After moving several old large movie projects from the source drive to another drive, I decided to delete the corresponding files from the clone drive.
At that point, I discovered the cause of the nightly Smart Update space failure:
- The day before, I'd renamed the 112 GB folder containing movie projects.
- Smart Update had partially copied the renamed projects folder when it ran out of space because the original projects folder was still on the clone drive.
If Smart Update had started by deleting that original folder from the clone, there would have been space to copy the renamed folder.
I deleted the extraneous old folder from the clone drive, and last night's Smart Update ran successfully.