I was wondering if Cisco have release the fix for this problem. I encountered the problem a couple months ago.
"the bug causing the problem is documented as CSCsm31861. I have attached our case to that bug.
The workaround is to stop the daemons, and then do the backup manually.
So that backup problem is caused by DFM writing to a log file frequently (that's the bug that will be fixed in May). The way to get DFM to stop writing to the log file is to shut down the daemons while running the backup. I had thought there was more to it than that, but I was looking at a different issue."