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Here in Florida our "cold" days are few and far between. Recently, when the camera was cold ERR 99 poped up. Going through the usual routine, battery removed, card reseated, the problem goes away. This has happenned a few times and with both bodies. .
A little internet research comes up with several posts on other boards regarding the problem with Canon and Lexar, but they are almost four years old. After reading about the cold and memory cards, I placed the cards in the freezer for about ten minutes and all of the Lexar cards had read issues in both of my 1D mk III's every time the test was done.
Lexar is in the process of replacing 4 8gb 800x cards since it has only happenned with that particular brand and they are brand new.