In the past I’ve encountered times where I will open Adobe Digital Editions, connect my NOOK to my PC, and find that ADE doesn’t recognize it. Here’s the work around I’ve found that works:

Open Adobe Digital Editions.

Connect your NOOK to your PC with a USB cable.

If you’re on a Mac, you’ll have to have the Android File Transfer Tool installed and open it.

If you’re on a Windows machine, you’ll find the NOOK folder in Windows File Explorer.

With Adobe Digital Editions opened, and Android File Transfer Tool or your File Explorer opened; Right click on the title you’d like to move, and from the details menu click Open in Finder, or the equivalent on Windows — likely open ‘folder/directory”.

With the Finder window or File Explorer window containing the Adobe Digital Editions content in it open, and the Android File Transfer Tool or Windows Explorer window with the NOOK folder visible, drag and drop the content from the ADE window to the NOOK folder.

With that, I was able to see it in my library on my NOOK.