Canon EOS 1Ds Mark II Instruction Manual page 119

Handling "Card Err"
If "Card Err" is displayed on the LCD panel, it indicates a problem with the
memory card that is preventing the image data from being recorded or read.
Use another memory card instead.
Or, if you have a memory card reader (commercially available) that can read
the card, use it to transfer the images to a personal computer.
After transferring all the image data, format the memory card. It may then
return to normal.
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