reading AND writing an exabyte tape

[Russell J Fulton;ccc032u]

I have been trying to implement a system to enforce tape label checking for
our backup system. 
If I first verify that the operators have mounted the right
tape by reading the label which has been previously written on the tape and
then try to write to the tape I get an I/O Error.

If however I rewind the tape after reading the label and then fsf the tape
all works as expected. 

i.e. It would appear that one must rewind the tape when switching between
reading and writing. Is this behavior intentional? On other systems I have
had programs that read their way down to the end of a tape and then appended
data. 

Is this the reason why -a option of tar does not work?

Thanks, Russell.
 
-- 
Russell Fulton, Computer Center, University of Auckland, New Zealand.
<rj_fulton at aukuni.ac.nz>