I am using a lexar V60 250 MB/s card BUT writing to it is extremely slow, even though I have formatted it in camera. I recently took a sequence of 6 images of a train approaching using an APSC lens so the images were only 25MP (shot in raw) and I turned the camera off after the train had passed, not thinking to check the images had been saved in the previous 15 seconds - all the files appeared on the disk as unreadable whilst images either side and through out the disk elsewhere were fine. I assume if I turn the camera off to move on after 10-15 seconds it simply is too slow to save 6 25MP images??
