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.
So, assuming you have the requisites for this, first thing you'll need to do is navigate to your storage, and then to the slots on your tape drive.
From there, select the media that has the end marker unreadable error and click the arrow at the top-right of the BE interface to see the details of this media. From there, select the media sets tab on the left. You should now see all the media sets that were backed up onto this tape. Almost definitely, the one at the top of the list (the most recent) should be what threw the error. Select it, and click the arrow to go to the details of that media set. You should have two tabs to the left, Contents, and Properties. Properties is selected by default. Select contents, and verify that there is a red circle containing an X over that media set. That means that this piece of information has some form of corruption, you can navigate to the exact problem file through expanding, but, unfortunately, you can't just de-select only that one file, so you'll have to lose all the data on this backup set. If there is no red X, check all the other sets until you find it.
Now that you've determined what data is bad, select every other media set in the list other than the bad one, and click the duplicate button on the top ribbon. Once it loads, select your destination device, media set, and any other settings you need, and run that job. Once the job has been run successfully, you'll now be able to safely scratch and erase the tape, given that the data on it has been backed up elsewhere, thus removing the end marker unreadable error, and allowing use of all the space rendered useless because of it.
NOTE: It could be possible to back up the corrupt file too as far as I know, as I haven't actually attempted that, but given that I was down to only two tapes when I had to do this, I wasn't about to risk making my other tape unreadable as well. Feel free to try it and let me know if you have the open resources to do it.
Also, I'm by no means a professional of backup exec, so if anyone sees anything wrong with this method that could be problematic, let me know. This is simply a method to be used in extreme circumstances, when the free space is absolutely necessary. More or less, as a last resort.
EDIT: I uploaded a visual guide as the featured image, however, I'm not seeing how to access it. It's too large to insert as an image, and when I split it up into 6 smaller, uploadable files, it was compressed automatically to an extend that the text in the guide was unreadable. I would use imageshack, but, of course, the web block here at my work won't let me use that site. Anyone know how to make the feature image accessible, or overcome the automatic compression?