Error when trying to change storage mode

I encountered an error when i tried changing the storage mode to external. I have a mybook. 1.5tb do you think it could due tot he fact that it mounts two things when you connect it? It was copying over for 2 hours until i finally exited out. help?

That sounds normal. Depending on how much media you currently have on the AppleTV it may take up to 4-5 hours for the initial transfer to complete.