That link isn't working for me but I'll try to format the card with a PC and see what happens... Thanks for the pointer.
Link quoted below:
16GB SD Card Formats to 8GB in Camera
February 17, 2012
I have a Canon EOS 20D and recently I started using SDHC cards for storage in combination with an SD-CF adapter. Works great and SDHC cards are 1) pretty cheap compared to CF cards and 2) work with all the other devices I have that support SDHC, including my MacBook Pro. I got a pair of 16GB cards and it’s pretty hard to use them both up on long trips. I ended up using only a little over one card on my recent two week vacation where I shot about 2000 photos in RAW+JPEG mode.
Anyways, I was clearing out a card so I could put some new photos on it and I decided to use my camera’s format option rather than doing an erase all, thinking it would be faster. It might have been, but it formatted the card to an 8GB capacity – half of what it should be. Turns out that while my camera can handle 16GB cards, it can only format 8GB ones. Popping it back into my computer to format it didn’t help either, as it now only recognized 8GB as well. Some hunting around the Internet led me to this blog post(1), which suggested reformatting the card with this USB_Format utility(2), which worked like a charm and I have 16 GB available on that card again. Now I just have to remember not to format this card in my camera again…
(1)
http://davidnaylor.org/blog/2008/04/problem-and-solution-16gb-compact-flash/(2)
http://www.captiveworks.org/files/usb_format.zip