Don't know whether it's a one off, but when using the Lexar 1000x 32gb CF card for the first time to day, the card lost contact with the camera.
After taking about 100 shots on the CF card I noticed my 1dm4 had transfered the data saving to the SD card slot. I was able to select the CF card again but as soon as I shot the camera started using the SD card again. I was not able to play back any of the images on the CF card.
When I ejected the CF card and reinserted it, everything seems back to normal.
This is the first time I had problems with a CF card, but I have previously only used Sandisk Ducati's and Extreme Pro's, so it's not confidence inspiring.
The card was formatted in the camera, so I can't understand how it could loose contact.
All the other sandisk cards have worked in the 1dm4 without a hitch for years