Make sure you are using the lastest device drivers from Symantec. Your tape library was only recently supported. Here is the link:
Symantec Backup Exec (tm) 11d (11.0.7170) - Device Driver Installer (20070415) - Includes Tape Driver Release (28) and Robotic Library Release (76) (x86-32 bit)
http:\\seer.entsupport.symantec.com/docs/288038.htm
Go to Windows Device Manager and make sure there are entries for both Medium Changer and Tape Drive. If the Medium Changer entry do NOT say "Unknown Medium Changer," then you will need to set it that way.
CHANGE TO "UNKNOWN MEDIUM CHANGER"
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1). On your desktop, right click the “My Computer” icon
2). Select “Manage” then click on “Device Manager”
3). Click on the [+] (plus sign) to the left of “Medium Changers”
4). Right click on the HP medium changer listed and select “Properties”
5). Click on the “Driver” tab
6). Click on the “Update” button
7). If the Wizard asks to connect to Windows select “Yes, this time only” – then click NEXT
8). Select “Install from a list or specific location (Advanced)” – then click NEXT
9). Select “Don’t search I will choose the driver to install” – then click NEXT
10). Select “Unknown Medium Changer” – then click NEXT
11). Follow the rest of the on-screen prompts until you are finished