I know there a lot of questions about Data Protection Manager (DPM) 2007 and Quantum DLT-V4 configuration. Here are some useful information if you have trouble with confirming the tape drive.
Make sure you install all latest DPM hotfixes.
Update tape drive with latest drivers.
If the DPM job still failing, you can try the following method:
1. Go to Computer Manager-> Storage-> Removable Storage-> Libraries and select Quantum
2. From the right pane, right click on tape select Free the tape option.
3. Open DPM Admin Console
4. Go to Management –> Libraries
5. Run Inventory
6. Right-click the tape and select mark tape as free option