Well, I've been using an Exabyte 8200 on NetBSD for years.  This is
under I386, but I think the drivers are similar enough that you shouldn't
have a problem.  Right now I'm backing up 1.2G on my system every week with
room to spare on the tape.  For what it's worth, here's a script of what
parameters I'm using. It's also worth noting that we use these parameters
un SunOS connected Exabyte 8200 machines as well at work where they perform
the same as they do on NetBSD.  I think the dey is the density and block
size.  The default block size lets the 8MM tapes waste a lot of space.  The
larger block size keeps the tape streaming and the blocks full.  I wonder
if you convert to these params if you  have the same trouble?

Good luck.
-Brian


# doing level 0 backups for myhost
# backing up a total of 1221.96 MBytes
# If that is more than 2000, the dump will likely fail.
#
rdump 0fubsd /dev/nrst0 100 6000 54000 /dev/rsd0a |& tee -a /tmp/log
rdump 0fubsd /dev/nrst0 100 6000 54000 /dev/rsd2c |& tee -a /tmp/log
rdump 0fubsd /dev/nrst0 100 6000 54000 /dev/rsd0f |& tee -a /tmp/log
rdump 0fubsd /dev/nrst0 100 6000 54000 /dev/rsd1e |& tee -a /tmp/log
rdump 0fubsd /dev/nrst0 100 6000 54000 /dev/rsd0e |& tee -a /tmp/log
rdump 0fubsd /dev/nrst0 100 6000 54000 /dev/rsd1g |& tee -a /tmp/log
rdump 0fubsd /dev/nrst0 100 6000 54000 /dev/rsd1h |& tee -a /tmp/log