I have a new EOS Rebel T3 and I purchased a new 64gb SanDisk SDXC card to go with it. I formatted the card while in the camera before use, but when I inserted it into my laptop to download the photos, a message popped up on my laptop and said I needed to format the card. I was able to download the photos using the cable from the camera to the laptop. After I did that, I formatted the card on my laptop just as I was prompted to by my laptop. When I put the card back in the camera, it said it was full, even though I just formatted it. I re-formatted the card on the camera, but my laptop keeps telling me to format the card - what am I doing wrong, if anything?
the 64GB cards use a file system called exFAT. if you are using Windows XP you need to install a patch so it can recognize exFAT partitions.
here is a knowledgebase article with the patch info