...on my G3.
It always annoyed me how I could never get my 420EX Flash to work on my G3 during the day as a fill flash, the internal would work fine, but I prefer the 420.
Today, as usual I went inside after doing some action shots (key point here, the camera was in 'rapid fire' mode) of the 1 year old, and went to use the flash for fill, and guess what no go.
I then put the camera down, and it powered itself off after a couple of minutes, I then went to shot another shot, without removing the flash, assuming it wouldn't work, and guess what, it fired!
The difference, when you switch the camera on for the first time, it goes into single fire mode, and unlike the internal flash, an external flash will not operate if the camera in on 'rapid fire' mode.
So there you go!
