We have the same library/drive combination.  Are there tapes in the drives when
they get downed?  Are you using library-controlled cleaning?  The scenario I've 
seen
(and which has been reported by StorageTek - manufacturer of the 20/700 
library) is
as follows:
 - tape drive signals that it needs cleaning
 - the library notices the "clean me" signal and schedules cleaning at first 
opportunity
 - library places cleaning tape into empty, dirty drive w/o notifying NetBackup
 - NetBackup attempts to place a regular tape into the now occupied drive
 - drive immediately gets downed due to unknown tape in the drive
 - when cleaning is complete, library removes cleaning tape and you're stuck
   with a down drive

Check this entry for an excellent discussion of the problem:

http://mailman.eng.auburn.edu/pipermail/veritas-bu/2001-February/002550.html

TapeAlert mentioned in this article is supposed to be "fixed" in NB 3.4 but I 
cannot
verify this as we're still at 3.2.  There is also a script available which ups 
downed 
drives automatically.  See:

http://mailman.eng.auburn.edu/pipermail/veritas-bu/2002-January/007278.html

--
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