I had encountered the same error on a Powervault TL2000 as well. opening up the library showed the same symptoms, the picker was unable to move up and error was returned.
After dismantling the picker and looking at the picker closely (i manually used my fingers to rotate the gears to move picker up/down). i realised the movement wasn't smooth at all with a snag hitting a certain spot before the picker was able to move further up/down.
to cut the long story short, the problem turned out to be a cracked gear. Since the gear had cracked, it became slightly larger and thus causing the snag when the attempt to move up/down is being processed.
I ordered a pack of 10 similiar gears (12 teeth mod 0.5) from eBay for about USD3. the original gear that the tape library used has a internal diameter of just under 3mm and the ones i ordered are 2mm. So I had to use some torx screwdriver bits to slowly enlarge the internal diameter of my replacement gear.
after replacing the gear, the library still returned the same error. however, this seems to be because the picker was not in the original position as it was supposed to be since i had shifted it to the middle in order to remove it. after running an inventory check command, the picker was moved back to the correct position and the robotic error disappeared.
the library has been running fine for about 2 weeks now with no other issue. a USD0.30 fix for a 10 grand machine.
some pictures below. (warning HUGE images)
image showing where the gears are (black are original gears, white is replaced gear)
http://img521.imageshack.us/img521/6578/img5032mo.jpg
image of replaced gear
http://img818.imageshack.us/img818/6752/img5033mo.jpg
image of gear similiar to the original cracked one
http://img15.imageshack.us/img15/2109/img5034fm.jpg