Sometimes you may with to add files to an existing backup; if you issue a command like:
tar -cvf /dev/st0 backupfiles
…and the tape is not already set to the end of the previous archive you will over-write any data from the position on the tape. Use the “eom” command to move the tape to the end of the alread-recorded files like so:
mt -f /dev/[path-to-tape] eom
mt -f /dev/st0 eom
Now you can use tar to add a file to the tape without over-writing the existing data.