SmartUpdate failed: target disk full

[macWish]

My schedule smartupdate of mu User Files failed for the first time ever last night. The log showed the following final message:
" 10:04:23 PM | Error | ditto: /Volumes/Backup/Users/myname/.Trash/21-31-03-Default.3gp: No space left on device"

When I checked the size of the target (a 29.8 GB partition on my external hard drive), it showed that there was 0 free space left and 29.7GB used. However, the user folder on my internal hard drive that I was attempting to copy with SuperDuper! is only 10.7 GB and the total used spaced on the internal drive is 24.1 GB. I am at a loss as what SuperDuper! copied to the external backup partition. When checking the backup, I am not able to find what is filling up the extra 29 GB. Any suggestions other than erasing the backup partition and starting again?