I’ve been using SanDisk Extreme III cards on my cameras for a long time, downloading the files to my Windows XP machine with no problems. I just bought two Extreme Pro 64 cards, formatted them on my camera, and the shots look good on the viewfinder screen. But when I plug them into my PC (with a Sony MRW62E card reader), it does not see the card. BUT my Mac Pro downloads the files just fine. Are Extreme Pro cards not compatible with Windows XP? Will they work with Windows 7?
Your Windows version may need to be updated, or a different card reader to see the SDXC (exFAT) format of the higher capacity cards