Check the library's status via robtest, see if a tape can be mounted:
/usr/openv/volmgr/bin/robtest

select library

s d (show status of the drives)
s s (show status of the slots)
m sN dN (moves slot 1 to drive 1)


Check to see if the OS can read the tape:
/usr/bin/mt -f /dev/rmt/Mcbn status (may differ from N, make sure to check what N is compared to M back when you first started robtest)