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On rebel or xx0D series cameras by Canon, the way the camera can tell if a flash is on is due to a sensor on the hot shoe itself. Underneath one of the side rails is a little bar. When a flash is off, this bar goes from the bottom to the underside of the rail. When a flash slides in, this bar is pushed down then informs the camera there is something attached.
When you remove a flash, this bar goes back up and tells the camera there is nothing there.
However, since this is just a thin piece of metal, and if you use your external flash often, it may not come back up as high as needed. When this happened to me, I just took some pliers and gently pulled one end up more and the built in flash started to work again.
Let me know if any of this needs more clarification.
_________________ Canon Rebel XTi, BG-E3 Grip Canon EF 16-35mm f/2.8L II, 24-105mm f/4L, 50mm f/1.4, 70-300mm f/4-5.6 DO, 85mm f/1.8 Sigma 150mm F2.8 EX Macro Sigma 10-20mm f/4-5.6 EX, Canon EF-S 18-55mm f/3.5-5.6 II Canon 430EX II Opteka 13mm, 21mm, and 31mm extension tubes
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