Hi all..
I recently shot part of a low budget movie using the canon C300.. I really liked the camera and of course the pictures are unbelievable.. But I believe I may have uncovered a small bug..
I was recording to 2 CF cards at once ( dual record mode ) This all work well except for when I changed the frame rate to 3 frame per second.. Any clip shoot at 3 frames was only recorded to one CF card.. Over 8 days I shot at 3 frames on 3 separate occasions.. the first time the speed up clips were only recorded on the first card.. the second time they appeared on both cards.. and the last time the 3 frame clips were only recorded to the second CF card.. All very confusing.. I admit I have not read the manual from cover to cover so perhaps I have missed something..
Can anyone shed any light on this..?

Whenever I switch to Slow & Fast, dual recording is disabled. I figured this was normal operation.

See page 44 of the manual, Selecting the CF Card Recording Method:

"Double slot recording cannot be used... with slow & fast motion recording."

OK.. AS I said I have not read the manual from cover to cover.. so thanks.. However ..this still does not explain why my slow and fast footage was not recorded consistently on just one card.. Sometimes slot one and another time slot 2..

A couple of possibilities:

If a card slot door is opened during recording, then the recording automatically jumps to the other card slot.

If you tried to record fast motion video (3fps, like you said) to both cards, instead of returning an illegal operation error, the recording system simply "picked the best slot to go to at the time," in one case it was CF #1 and another case it was CF #2, but this is just my own theory.

Also, at such a slow frame rate, the recording system may have decided that "just this once" it could lay down 3fps to both cards simultaneously, but its inability to do that consistently is the reason why the manual states that it's not possible. Again, just my theory.

Long story short: it's not a bug... it's a feature: dual recording is disabled during slow & fast motion video.