I had that problem when I first opened my 600 and was trying to copy books into my reader that I had purchased before it arrived.
After the 3rd time I tried (I'm a slow-but-steady fixer), I went out to the Sony ebook site and "authorized" my Sony reader before trying to transfer the books.
I have not had the problem again, and suspect some kind of conflict with a computer "not authorized" message - but I'm pretty technologically behind the times
when it comes to all the wonderful techs here.
Good luck!