I have had to do this twice now. I download a copy of my realm to get stuff set up to re-upload it later. After working in it for several hours I saved and quit, then closed the game and shut off my xbox. Upon returning to the game to continue working on the copied world, I found that it was no longer in my list of worlds. I had to re-download it and do all of the work over again (very frustrating). It has now done this to me again and I will have to repeat the process over.