I still continue to have this problem on some but not all epub books I borrow from my library and copy to my Nook. I have no idea why some work and some don't. The file is absolutely ON THE NOOK but it will not show up under my files or my library.
I ended up installing calibre (
https://calibre-ebook.com/) and copying those problem books that way. I have to download the file from the library, open it in Adobe Digital Editions so that it authorizes it, and then use calibre to copy it to the nook. It's a pain but it works....so far!