Originally Posted by
Tannin
So this is with the connected internal flash and the external flashes switched all OFF? (Just to confirm.)
If so, then the camera is detecting the flash where it should not. When you attach an external flash to any Canon camera (within), if it is switched off, the camera normally detects nothing (which is correct) and functions exactly the same way as it does with no flash connected at all. This has been true of however many different Canon DSLRs I have used with external flash, maybe six or eight models, certainly including 7D and 7D II, 1D III and 1D IV, 5D II and 5D IV, and probably one or more of 20D, 40D, and 50D as well. (The flashes have all been 580EX II or 600EX some model I forget. Does 600EX II RT sound right? Doesn't matter, in this regard they are all the same.)
Just one thought - OK, it's a really stupid question but not as stupid as not asking it - you are physically switching both the camera and the flash off and then restarting them in the correct order whenever you change your gear around? 1: switch everything off; 2: add or remove flash; 3: switch on flash(s); 4: switch on camera. If you fail to do this, things quite often work anyway, but you tend to get particular niggles. The one I usually get is the camera not recognising the flash correctly and thinking it is a dumb flash not capable of HSS.
From memory, this happens more often with some bodies than others. I think the 1 Series ones are less fussy than the old 5D II, but I can't remember exactly.
Another thing to try to troubleshoot: borrow different Canon flashes to test with. Confirm that the problem is still there. And what happens if you remove the batteries from your flash? Does the camera still go to 1/250th? Have tried cleaning the contacts?
Sorry for so many different, disjointed points all jumbled together Paul. It's late and my brain has turned into jelly. Bedtime!