Same situation. I'm also on a One X and am hoping to tap into the expanded storage. Hopefully someone can reply with a work around.
Edit: @Moraduke I got mine working as a Game. I think the trick is to launch RetroArch from the Home view once you've switched it to Game instead of trying to launch it from the dev mode view. Hopefully that helps you out.
Thank you for the reply! Thank goodness I'm not the only one. This seemed to work...at first. I was able to exit out of Retroarch and go back into it. Then I updated the assets and quit out of Retroarch and now I'm right back into took-to-long-to-startopolis. Either something got corrupted in my Xbox's file system somewhere where it cannot cache Retroarch or it doesn't know how to cache it in the first place. Either way, this is bizarre and frustrating.
I guess I could try factory resetting my console which is a PITA, but I just want to play more of the Dreamcast version of Ready 2 Rumble Boxing and possibly get some PS2 stuff somewhat working.