I know this has been discussed in the past, but I thought I would remind everyone how to do it.
Everytime I back up with DM, I get an "Unknown error" when the backup gets to "Backing up on-board device memory".
The fix is simple:
Go to Options:Memory:
Turn off "Mass Storage Mode Support"
Escape out and SAVE your changes.
Backup as normal. Re-enable Mass Storage Mode Support and you are good to go.