On a media server with one DLT drive I see the above
message, even though there is a tape mounted in the drive
and it has been just written on; the tape is not being
unmounted, so this is not the cause. Because NetBackup
thinks there is no media in the drive (at least this is
how I interpret the above message), it tries to
allocate/mount a volume which it has in the database and
of course this simply fails. (These backups are
unattended.)

Any suggestions as to what might be causing this?