Originally Posted by Dave W
So far, I haven't figured out how to do that with the Nook. I'd rather not get into stripping the DRM in order to do that. I didn't do that with the Sony.
If you try to add a DRMed book to Calibre, it will refuse to do it. So, if you want to use Calibre to control your ebooks, you have no choice but to install the DRM-removal plugins. Otherwise, Calibre just will refuse to add the books. I do not know what it was about your Sony books that allowed Calibre to use them, but apparently they did not have DRM.
Removing DRM to allow yourself to use Calibre for your purchased books and/or to back them up is a perfectly valid use of DRM-removal, IMHO. Piracy is NOT a valid use, IMHO.