You are trying to append to a tape. That can't be done. Here is the output direct from the man page.
option -r Add the named file to the end of the archive.
The same blocking factor used to create the archive must be used to append to it. This option cannot be used if the archive is a tape.
You will probably have to create your TAR ball on disk and then later copy it to tape when you have your complete TAR ball.
You probably don't have a problem with the blocking factor, but since you are trying to append a tape that will be a show stopper.
