If the tapedrive is "DOWN-TLD", you could use the UNIX "mt" command on the device path...
mt -f <device_path> status
Example: mt -f /dev/rmt/0 status
If the OS believes there is a tape in the tapedrive, it will report some vendor sens cod...
If the backup runs for a bit, it is selecting one of the tapes to write on; once it hits the end of media mark it is forced to grab an AVAILABLE tape (not an ACTIVE, partially full, tape), of which you have none... and the backups fails. Status code...
When no backups are running, no restores are running, and no Pending Requests...
On Master Server
cd /usr/openv/netbackup/bin/admincmd
./nbrbutil -resetALL
stop Netbackup
start Netbackup