mt -f /dev/rmt/**

Something very wrong at OS level with devices?

Forget about NBU for now and fix access between OS and devices.

write protected or reserved means one of the systems accessing this drive is holding SCSI reservation on the drive.
If SSO environment, 'vmdareq' output on master might tell you which media server has RESERVED status on the drive.

To release reservation, try the following:
mt -f <drive_path_name> forcereserve
followed by:
mt -f <drive_path_name> release

No such file or directory means the OS has lost connectivity to device - either physical path problem or lack of persistent binding which results in device names to change when server is rebooted.

To rebuild OS device paths, issue
devfsadm -C -c tape
 

no tape loaded or drive offline is normal status for empty drive.

For more info on mt drive status, see man pages of mt command (man mt).