I found a couple of items in the tar info manual that might apply:
`--append' cannot be performed on some tape drives, unfortunately,
due to deficiencies in the formats those tape drives use. The archive
must be a valid `tar' archive, or else the results of using this
operation will be unpredictable. *Note Media::.
In order to update an archive, `tar' must be able to backspace the
archive in order to reread or rewrite a record that was just read (or
written). This is currently possible only on two kinds of files: normal
disk files (or any other file that can be backspaced with `lseek'), and
industry-standard 9-track magnetic tape (or any other kind of tape that
can be backspaced with the `MTIOCTOP' `ioctl'.
This means that the `--append' (`-r'), `--update' (`-u'),
`--concatenate' (`--catenate', `-A'), and `--delete' commands will not
work on any other kind of file. Some media simply cannot be
backspaced, which means these commands and options will never be able
to work on them. These non-backspacing media include pipes and
cartridge tape drives.
Some other media can be backspaced, and `tar' will work on them once
`tar' is modified to do so.
I don't know if this would apply with your hardware. Could you create another backup on the same tape?