We've been downloading books to our nook for years but recently keep getting error message "failed to write destination file" - why?

I had this problem as well, and after doing some research via google (because I found NOTHING helpful in adobe forums), I found the problem....my Nook was full.  I had no idea this was even possible!  But, I borrow a lot of library books, and didn't realize that after they expire, they are still taking up space on my Nook.  Here's how to check: plug your Nook into your computer, go to file manager (right-click the start button in Windows), find the Nook drive and right click, choose properties.  This should show you a pie chart of used/free space on your Nook.  If this is the case, you need to remove some files from your Nook.  For me, I had to find the Digital Editions file and delete all the thumbnails and actual files for the library books.  If it's booked you've purchased, perhaps you need to archive some books.  Hope this helps!