Shared tape library drives have red Xs against the shared users under the drives. (4043634)

From NetVault Device Management we could see that the drives and the tape library robot arm were green, but only the NetApp Filers under the Drives had red “X” against them.
With Clients under the Drives, this means that they are sharing the drives with the Filers and are classed as Dynamically Shared Drives (DSD).
From the Server License - Capabilities and Usage Report we could see that the drives were dynamically shared drives.
This was confirmed in the Binary Log too:

Severe Error 2011/09/13 10:46:30 0 Media PCH-NETVAULT Exceeding license for DSD(TM) (have '2' licensed for '0')) and are not licensed for this.

I confirmed that the Device Serial Numbers for both drives are the same on the tape library web interface and in NetVault Device Management, so this wasn't the issue.