I have a canon rebel xti with an 18 to 55 lens .. I recently bought an external flash to sit in the hot shoe .. now when I removed the flash from the hot shoe .. the cameras own flash will not pop up and work... I can manually pop the flash but it still will not work..  I thought that when I bought the flash at the camera store the sales person turned off something on the camera for the new flash to work.. is there something I need to change in the menus to get the cameras own flash to work.....

there is a sensor under the lip of the hot-shoe, you need a very small screw driver to lift up the piece that pushes down on the tiny button that tells the camera that there is already a flash mounted. this was caused by leaving the external flash on too long. try it, good luck. the sensor button is tiny., may need magnifying glass.

It is not possible to use the Nikon SB-50DX directly on a Canon EOS Rebel camera. The Nikon flash gun hot shoe contacts do not match the Canon hot shoe contacts.
You can use the Nikon as a slave flash gun with a PC flash cord that matches the Nikon's flash contacts then connect the end of this flash cord to a flash trigger. The Canon's flash can then trigger the Nikon flash gun via the flash trigger.

No. The rebel, or any digital canon camera requires the use of the EX or EX II series flashes.

Be aware that the built in flash does not pop up automatically in all picture modes. Try it when in the different modes (on the mode dial) and see what I mean. Does it popo up when manually opened ?, (small button on front left hand side of the viewing pentaprism/flash housing.)

err 99 means there is moisture inside of the camera, probably inside the hot shoe

Hi Chrissy, I had observed the same problem - after an external flash had been removed from the camera, internal flash didn't pop up. Tried to look at the Google and finally found the solution. It helped me and could help you as well:
The usual reason is that the camera thinks that there's an external flash mounted on the hot shoe. Check the external flash detect switch. It's located under the right side of the shoe (there is a metalic piece which is kind of "elastic". Just try to push it and release several times, may be put the screwdriver softly under it and out... It helped me. Here is the picture of location of the detector.

I also own a Vivitar 285 that i used with my Canon F1 manual 35mm camera. Most older cameras before the advent of digital, had a hot shoe on top of the camera, or like in my case, the hot shoe went over the rewind knob. I also had to plug in a power cord from the 285 flash foot and the cameras PC connection in order for the flash to work on some other cameras. The Canon Rebel XTi has a TTL (through the lens) hot shoe usable with Canon's own EX flash units. You can buy a "hot shoe to PC connector" if the TTL camera hot shoe doesn't work, but be extremely careful, the only problem that exists with the older manual flash units was the trigger voltage is as high as 200+volts which would fry digital camera hot shoes. I tried my unit with a Wein flash trigger device on the PC cord of the flash and my Canon D60 with it's own flash. All shots were overexposed and washed out highlights.  Here's a copy of the hot-shoe review:h The EOS 400D's hot-shoe can be used with Canon and third party flashes (although sync only on most third party units). The hot-shoe is E-TTL II compatible. Compatible flashes include Speedlite 220EX, 380EX, 420EX, 430EX, 550EX, 580EX, Macro-Ring Lite, MR-14EX, Macro Twin Lite MT-24EX and Speedlite Transmitter ST-E2.  The Canon Rebel XTi doesn't have a PC socket to plug in a manual flash trigger cord which common sense tells me there is no provision to step up the voltage to trigger manual flashes - so my answer is this combination is not a good idea on a Canon Rebel XTi. Hope this helps.

sounds like you might have an overheating CCD. Turn off the camera for couple of hours and when you try again if this happens again, return it to the store you got it from for warrenty repairs.

I found the answer and fixed my flash. Look on http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00ODWf

This will describe how, if you have been using a speedlite alot, than the little tab under the shutter release side of the hot shoe can get bent down thus constantly pressing down (toward the bottom of the camera) the small button which deactivates the pop-up flash even when the speedlite is not attached. Just bend the tab up a little with a jewler's screwdriver, turn the camera off and on and voila!! Fixed!!! Dan describes pushing the tab down, but on the xti, the tab needs to be bent up to release the flash deactivation switch and fix the problem. Thanks to Dan D on photonet for saving me a service call.

ALL Canon digital EOS cameras will work with E-TTL only. Solution: you need EX flash for full auto.