What is wrong with my dstt MicroSD cards?

I have a dstt cart for my DSi. I currently have a 4GB MicroSD card to hold my games, but I recently came to possess an 8GB card. However, when I try to edit either card on my computer using the USB adapter, it says I have to format the disc before I can do anything. Reluctancy aside, even when I try to format, it says Windows cannot format. What do?