Adobe Digital Editions 3 . Transfering book to ereader

Hi. Can anyone help please. I am having trouble downloading my ebooks onto my e reader. I am using Adobe Digital Editions 3 and when I try to move the book from my Adobe library into the reader I get a fault message "Failed to write destination file." I have been using this system for many years without any problems but this problems has just started. I have tried reinstalling ADE3 and it has not fixed the problem. I am buying books from KOBO and the ereader is a NOOK. Thanks.