First i should metion that there is no reason to backup /dev. The files in /dev are device files with no usable content. They are created depebding on the devicees installed during boot.
Also you should you the syntax
Code:
tar cvf /dev/nst0 $filespecification
to find a file on your tape. The parameter "f"define the device or the tar file which you are trying to get the table of content from.