I'm not sure if anyone has made this observation before, but I didn't see it in the forums, so I"m just going to post it in case it's useful to anyone.
This is a way to remove an end marker unreadable from a tape, without losing all the data on that tape. It's not perfect, and you will lose some of the data, but it's a method that can be used if you're very low and tape space and need a tape that, say, only got 5% written before it became unappendable due to an inconsistency, network error, etc, throwing an end marker unreadable error.
To be able to do this, you'll need to have at least one other writable tape, or disk storage, available with enough space to contain all the data on the tape that has the error. If you're going with another tape as your approach, you'll have to have a tape drive that can mount two tapes simultaneously. Otherwise, you'll have to write to disk storage first, and then to the other tape.