From your session report, it appears that your media agent is running on a HPUX system
On your cell manager, stop Data Protector.
At the command prompt on the Media Agent 'exmbx002', see what the status of this drive is
mt -f /dev/rtape/tape3_BESTn status
I'm not sure if this will show the drive as being offline or online
At the command prompt on the Media Agent 'exmbx002', do a
ps -ef | grep tape3
If you see any hanging processes, kill them.
Of course, you can also take the bruteforce method of cycling the power on the Tape Library. Turn it off for about 30 seconds, then, turn it back on, unless this is a OS issue, this should clear the problem
On your cell manager, restart Data Protector, and run
omnidbutil -clear
omnidbutil -free_locked_devs