This just started happening to me 2 days ago when I was trying to shoot 4k outside in 90 degree heat on a tennis court...so i thought it was simply the heat. I was getting the buffer symbol sometimes so I thought maybe the heat was slowing down the camera's ability to write 4k to the Cfast 2 card. Now I am indoors in 70 degree temps and its writing to the card for 4 seconds and giving me this message. I have not been outside or used the camera in 2 days so it is at room temp. Its doing this on all 4k fps settings. It did this to me on 1080p 60fps as well after 90 seconds. I am using the new battery that came with the 1dx mk2 as well. The card is the SanDisk Extreme Pro 64 GB CFast 2.0. I shot a wedding ceremony with it in 4k 30fps just 3 weeks ago outside and it filled the card up without turning off. What might be the issue? What could have changed. I formatted the card in camera in case file size was an issue due to formatting format.