There is a small micro switch in the hotshoe that occasionally sticks, making the camera think an external flash is mounted. As you look at the back of the camera, the switch is on the right side rail I think about 1/3 from the front. Get a tooth pick or very small screwdriver and move it around so it pops up.
I was going crazy trying to figure out what the heck was going on with this 60D body. Actually, I couldn't see the microswitch on the hotshoe rail, but by "flicking" the spring strip inside the rail with a toothpick, I was able to get the camera to stop saying there was an external flash attached.
Thanks for your post.