SirusVirus

Had the exact same issue with my X-T20 and SANDISK 64GB ULTRA PLUS (90mb write speed, microSD card with adapter) card. So 4K recording was working good, until some time, then the problem begun and after 9 seconds after start it was giving "write error". SOLUTION I simply removed the card, put it back. Removed the battery and immediately turned on the camera (yes without battery). Then turn off the camera. Put back the battery and card, and voila! It is working again. Hope this solution will work for you (just registered to write this).