Try using rdump:
# rdump Ouf hostname:/dev/st0 /directory

If that doesn't work try moving everything to the
Ubuntu pc and use command 'tar'.
Or make a tarball of the directory you want to backup and then
transfer it to the Ubuntu box. Perhaps you will have to make
several tarballs as not everything will be able to fit on a tape.

I'll give this a try. I can get the drive to work locally; it's
over the net that has me puzzled...