1DX MK II Video on Slot 2 Auto?

Is there a way whyen shooting video it flips to cfast slot 2, I can see myself forgetting to switch it over?

I see no option to do that but thought I would ask anyway.

No, there's not. You can shoot video to any card. It's just that if you go with the highest data rate, it will only run for a few seconds or so. But for lower resolution it works fine to record video on a CF card as well.

Page 153 user manual..

Movies cannot be recorded simultaneously to the CF card and CFast card. Movies will be recorded to the card set for [Playback].

So if you select the cfast for playback it is possible, providing you are writing to both cards simultaneously

You don't have to write to both cards simultaneously for that to work. Video will end up on the card used for playback, and that's it. There's no other option. Still photos will also end up on the card specified for playback, if you record to only one card.

Thanks, my thought is to write to CF card for stills and keep the video on the cfast, any downside to this other than remembering to change the slot?

And also for 4K what should the default frame rate be? So many 4K options here...

Higher frame rate is mainly useful if you want to show the result in slow motion, or if you have a particular inclination to burning through memory cards at a high rate.

As far as frame rate is concerned you may want to chose 30 fps, or close to it, for the US and other countries that use 60Hz mains. The close to 25 fps is better suited to use in countries with 50Hz mains, as we have in the UK and the rest of europe. If you shoot at the matching frequency to the mains you reduce the chance of seeing flicker in lighting etc in scenes.

Generally the frame rates being slightly below half the mains frequency is a consequence of how old school analogue TV worked. You have to allow some time for the dot to get back from the bottom of the screen to the top. It is also affected by the available bandwidth for each TV channel, this affects PAL and NTSC differently, even if both were on the same mains frequency.

Alan