We're running on a Sun E450 with Solaris 2.6 and NetBackup 3.1.1
with the latest patches from Sun.  We also have a Qualstar 46120
with 3 AIT-1 tape drives.  

We're seeing "(84) media write error" periodically in NBU.
It happens to any one of the 3 drives ... resulting in the tape
drive being taken off-line (down in NBU).  And it happens on
older media and newer media.  We initially purchased 60
Sony SDX-T3N tapes about 2 years ago and another 60 SDX1-25C
tapes about 6 months ago.  Both tapes are the same length and
capacity.  All media is in the same volume pool.  This configuration
has been working for over a year.

More clues form /var/adm/messages ...

  unix:  Error for Command: write     Error Level: Fatal
  unix:  Requested Block: 20547       Error Block: 20547
  unix:  Vendor: SONY                 Serial Number:
  unix:  Sense Key: Media Error
  unix:  ASC: 0x52 (cartridge fault), ASCQ: 0x0, FRU: 0x0
  tl8cd[24771]: TL8(0) move_medium error
  tl8cd[24771]: TL8(0) Sense Key:  Illegal Request (5h)
  tl8cd[24771]: TL8(0) ASC 3bh, ASCQ 90h
  tl8d[22465]: TL8(0) drive 3 (device 2) is being DOWNED,
       status: Robotic dismount failure
  tl8d[22465]: Check integrity of the drive, drive path, and media

And here's the st.conf entry (from this mailing list) ...

  "SONY    SDX-300C", "Sony 8mm AIT", "SONY_AIT";
  SONY_AIT = 1,0x34,0,0x9639,4,0x13,0x0,0x8C,0x8C,3;

We've done some testing but haven't concluded anything. 
Any ideas would be appreciated.  Thanks.

-------------------------------------------------------------------
Gary A. Moro                               email: gary AT cse.ucsc DOT edu
UNIX Systems Manager                       phone: (831) 459-4538 
School of Engineering                        FAX: (831) 429-9820
UC Santa Cruz
Santa Cruz, CA 95064