I use to see this occasionally with my 10.1d and 11d when it came to B2D devices.  Sometimes stopping and restarting the services, agents, and even the server(s) themselves would fix it but often times it wouldn't.

In my case, I found that whenever this occurred it was creating a new BKF file (which is the case when it can't overwrite or append to an exsistig BKF file as per the media rules that were applied to it) and for whatever reason it stalled when it started to create the file.

The way I would fix it (at least in my environment):

1. Go to the device view in the admin console and take note of the BKF file
2. Cancel the job
3. Associate the BKF file to the retired media set and then delete it
4. Navigate to the actual BKF file on the server and delete it (in my case it will be zero in file size)
   
5. Inventory the folder and then retry the backup

Again, this has been the fix that I've done with respects to my environment.  I do not have any tapes in my backup, just disks.  I do rotate my media/backup to an offsite location by way of hot-swap drives in an array.

For me, this symptom happens every now and then.  In other words, perhaps once every two or three months if that.  I have no explanation as to why it happens or why I need to do the steps above to correct the condition.  At first I suspected a corrupt or bad hard drive but I ruled this out a long time ago (I can go into detail but trust me that in my case it was not a bad drive :smileyhappy: ).

Hope this helped.