People often overwrite the beginning of large tar archive, leaving
  the first bit of the tape overwritten and the rest presumably intact.
  This is usually NOT recoverable.

  However, if you're feeling lucky, you might (says Dave Olson
  <olson@sgi.com>) try something like 'mt fsf 4; mt bsf 2; tar xe' or
  'mt fsf 4; mt bsr 2; tar xe'.  You might also try 'tar cv foo', where
  'foo' is slightly bigger than what you overwrote the archive with the
  first time, and pull the plug on the tape drive before it writes the
  EOF. Then power it back up and try 'tar xe'.

  If this sounds unlikely to work, you're right. Don't let it happen;
  use the write protect tab.