Both cameras that I own use CF's, my gps an SD...rule of thumb, format the card in the device that it will be used in. Your computer should see the card when you put into the card reader and format from there if you choose..and that is where you use it. But, I would try the device it came out first.
I have a 256mb CF card and I'd like to try to reformat it if possible. It's empty, but it's telling me that I only have 25mb available and I'm wondering if reformatting would help. I'm on a Mac by the way.