6D - Preview Unavailable For This File
had almost the same thing happened with my 6D when it first went on duty for a wedding coverage. it was a back-up camera so there is no worries. when i downloaded the files to my computer, at least 5 images (raw only quality setting) in different time capture was corrupted; but it showed okay on the LCD screen on the camera. I formatted the SD card (32GB Transcend Class 10) again and it never happened again. for me, i guess it is the card... i sure do hope so that there is no issue with camera as it is not used often yet but i tested the card with my sony nex-5n and 60d with no issues at all.
here is my post that was not answered: http://www.dpreview.com/forums/thread/3480503#forum-post-51375295
Just do your best at all times. Sick or not.