Here is an example of NetBackup Activity Monitor output when this error occurs:
3/20/2008 3:18:17 PM - media BR5002 required 3/20/2008 3:18:20 PM - restoring image barlnx1_1205947236 3/20/2008 3:18:25 PM - requesting resource BR5002 3/20/2008 3:18:25 PM - awaiting resource BR5002 Reason: Drives are in use., Media Server: bsrv3, Robot Number: 0, Robot Type: TLD, Media ID: N/A, Drive Name: N/A, Volume Pool: N/A, Storage Unit: N/A, Drive Scan Host: N/A 3/20/2008 3:18:35 PM - connected