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

  e.g.:  

mt -f /dev/st0 eom

  Now you can use tar to add a file to the tape without over-writing the existing data.