The "trick" here is understanding the different tape device files:
The second file is the 'non-rewinding' device. If you write to it, the tape does not automatically auto-rewind, allowing multiple archives to the same tape.
Once you go /dev/nst0, stick with it!
To move around the tape, use the 'mt' command, and REMEMBER TO ALWAYS USE nst:
mt -f /dev/nst0 fsf 1 # move forward one archive
mt -f /dev/nst0 asf 2 # rewind and forward to archive 2 regardless of current position
mt -f /dev/nst0 rew # rewind to beginning of tape
mt -f /dev/nst0 status # report where the hell on the tape you are.
If you slip up and use '/dev/st0' on any of the above commands, it will rewind to the beginning after completion!!!