I've set my 5D mk III to auto-switch between the CF and SD card when the CF card gets full.

I've went to the menu and selected "Record Func+Card/Folder sel". From there, I've set "Record function" to "Auto switch card".

My understanding is that when the CF card gets full, it'll switch, and begin saving photos and videos to the SD card... and this seems to happen.

However, what I've noticed, is that it'll also randomly switch from the CF to the SD card, for no apparent reason!

Both the CF and SD cards are 32GB. The CF card (Card 1) will be selected, and I'll shoot video and stills for a while... nowhere near filling the card.

I'll turn the camera off, remove the CF card and download the stills and video to my computer. With the camera still turned off, I'll reinsert the CF card (card 1), turn the camera on and shoot more stills and video... again, not filling the card

Sometimes, not all the time, but quite frequently, I'll go to dump the CF card, only to find that the images/video were saved to the SD card (card 2) instead! I didn't change anything, and the camera was off during the removal and replacement of the CF card.... so what's going on here? Anyone else experience this?

Anyone else experience this?

Yes, now that you mention it.

It's as annoying as heck! I never know where my footage is, until after I put the CF card in the computer, because I just assume it's going to be there!

Yep. It just happened to me today. Maybe we're doing something wrong.

I worked out what is going on!! Finally!!

If you are recording to the CF card, turn off the camera and remove the CF card... but don't close the CF card door, it'll select the CF card again when you put the card back and close the door. This is what we want to happen.

However...

If you are recording to the CF card, turn off the camera, remove the CF card AND close the door (as I do, to make sure it doesn't accidentally get broken off!), it'll select the SD card when you put next power on the camera. This is not what we want to happen.

The firmware should check to see if there's a CF card in the slot, and switch image destinations, when the camera is powered up... NOT when the door is closed! Clearly this is a design fault. Maybe we can petition Canon to fix this?