Some important things to note when using DPM. If you are told that the drive needs to be cleaned, you need to make sure that you put the right cleaning tape in there. When we got our new Autoloader (Quantum SuperLoader 3 with a DLT-V4 drive), we ordered all of the tapes we needed, including several cleaning tapes. Once the SuperLoader was loaded, we started to run with DPM and configured Agents and Disks and all the assorted sundries that a good backup scheme needs. (If you want to know more about setting DPM, I recommend this book)
Once things were setup, I noticed that the drive was reporting that it was in need of cleaning. Loading a cleaning tape through the mailslot, I followed the standard procedure of attempting to clean the drive. For some reason it kept failing, and I couldn't figure out why. After three or four attempts, I pulled the tape out and took a really good look at it and then looked at the packaging. Almost at once I saw that it was not the right cleaning tape; we had been sent the wrong tapes. Wanting to get things up and running we RMA'ed the bad tapes and overnighted the good ones. It turned out that they only had one of the tapes we needed in stock, so we had them ship it anyway.
Now that I had the right tape, I ran the cleaning procedure again, and the gnarly orange light on the front panel went away, and the LCD reported that all was well. This was not the case, however, as every tape operation I tried in DPM failed. Erase, Clean, Create Restore Point On Tape, all returned errors that the device was not ready. I tried rescanning. I downloaded the diagnostics from Quantum. I tried power cycling. Nothing worked.
As I dug further into the problem, I found that there weren't many people who had similar problems so as a last resort, I started to look up the actual error codes that I was getting. Most of them appeared to be generic errors, but when I searched for one particular error (24052), the first result from Google caught my attention.
Google Search on the Error
Looking through the post, it seemed that this person was having the same problems that I was having, but with different hardware.
Forum post that contained the information
It appears that this is not a problem with the hardware or the tapes, but rather a bug in DPM. For some reason, it looks like what is happening is that in the DPM database, when you try to clean using an incorrect cleaning tape (or the cleaning fails, I suppose), the "OperationOccuring" state in the "tbl_MM_Drive" gets stuck in a state (state 3, whatever that is) that prevents DPM from seeing that device as ready to use, even though in the Management tab, the device is being reported as idle.
While I hate to hack a database to fix a problem, that seemed to be the quickest way, so taking the advice of the post, I whipped out my l33t SQL skillz and ran an update to that row that changed the state to "0".
UPDATE tbl_MM_Drive
SET OperationOccuring = 0
WHERE DriveID = '<My Device's GUID>'
Once this code was executed, I re-scanned the Library in DPM and then erased a tape as a test. It worked, and I danced around the office, just a bit (everyone else was gone by this point).
While this is not a thing that most people would come across in the day-to-day operation, it is something that was almost a show stopper for us. Backups are extremely important- I've already used DPM to restore some accidentally deleted files -and for our peace of mind we need those tapes for off-site archiving. This fix took me a while to find, simply for the fact that ruling out the autoloader and drive takes a lot of time. Now that it's out there, I hope that anyone else who has this problem has a quicker time!