How do I move books from my HD internal storage to the SD Memory Card?
I tried responding to your question and Lithium apparently went crazy and deleted my original response.
Briefly, books purchased from B&N are stored in a 'hidden" partition on the HD and can not be accessed directly and "moved" to a SD card. They can be accessed through you B&N account and downloaded to a PC or Mac by using the Nook for PC (or Mac as appropriate) and "sideloaded" to the SD card. The books will remain on the HD and will still show up in your library. The storage space can be recoverd by moving the book to the cloud (press and hold the book cover icon and select move to cloud from the popup menu). The book cover will show a downward pointing green arrow in a white cloud indicating that it can be downloaded from the cloud. Now that I think about it, I'm not sure how it will handle a book that is stored in the cloud but also sideloaded to the SD card. Perhaps someone who uses the library and shelf functions can answer that - I personally rarely use the library and never the shelf functions.
Books from sources other than B&N should be able to be accessed using a PC/Mac connected to the Nook using the USB cable and manipulated like any other files once you have located them. You can also use a file manager like Open Explorer from the B&N app store or several others available from the Google Play Store to manipulate these files without connecting to a computer (some also permit wireless transfers between the Nook and computer).
One question that occurs is "Why do you want to move books to the SD card?" Epub format books are normally small files (1-2 MB), so unless you have a very large collection or have a lot of enhanced books, it doesn't seem to me to be worth the trouble of moving them simply to save space on the device (backing them up on a PC or separate storage device is a good idea even if they are in the B&N cloud). Magazines, pictures and video/audio files can consume a lot of space and are probably best stored on the SD card.
Hope this is helpful - good luck!