Ok so I am going to get my replacement body tomorrow (flew in too late to pick up on saturday). However I have just come back from a pre-wedding shoot this afternoon and the same flash related issue popped up again.
EF 70-200 F2.8 L IS USM II
Sandisk 60MB/s 8GB CF and Sandisk 90MB/s 32GB CF (i normally use match pairs but didn't have time to clear the full card prior to the shoot)
Shooting in broad daylight, Direct flash being used for fill, AV Priority, F4, ISO 200-800+ (HTP on), EC +1/3 and FEC -1 2/3, HSS.
Now in single shot mode it works fine. But i had the couple move around a bit so switched to AI Servo and this is where the issue begins.
The camera literally locks up - screens are powered, vf is powered but the lens doesn't rack in portrait or landscape, depressing either shutter does nothing, powering off the body does nothing.
Turning the flash off and turning the body off fixes the issue for a few frames but after a good 20 frames it repeated the issue.
-Switching lens to a 50mm and the issue didn't come up again.
-When this first happened overseas I was using the same configuration 600EX with 70-200.
-On a 7D this same config doesn't have the same issue.