I've seen similar problems when NetBackup is unaware that a cleaning tape is in 
the drive.  We use library controlled cleaning (L700) and occasionally, the 
library will decide to clean a drive and when NB attempts to put another tape 
into that drive, whamo, the drive gets downed.  Once the cleaning is finished, 
the CLN tape is put back where it belongs but you're stuck with a down drive.  
With verbose logging on all Media Manager daemons, you should see movement of 
the CLN tapes show up in syslog on your robotic controller which corresponds to 
the downing of the drive.  HTH!

--
Regards,

Ric Sokolowski (Ric.Sokolowski AT motorola DOT com)
Staff Systems Engineer
Phone: (954) 723-6332  Pager: 8776050503 AT skytel DOT com
Motorola, Inc.  / CGISS / Enterprise Computing
8000 West Sunrise Blvd, MS 22-2F, Plantation, FL 33322