I did the same thing, buying a My Book to pair with my 3tb My Cloud. I bought a 6tb My Book and connected it to my enclosure. It gets detected ok, and I can read and write to it. But for some reason, it makes the My Cloud lose connection to my router every few hours. I thought this was a problem with my new router at first and resigned myself to having to restart the My Cloud a few times every day. But then I connected the My Book to another pc and the My Cloud stayed connected to the router with no further problems. And when I reconnected the My Book to the My Cloud, I had to restart the My Cloud for it to work again. I have no idea why the 2 devices are having this issue. I would have thought that, if anything, they would be totally compatible for the exact reasons we are using them.