I was plugging the PCMCIA adapter into a PCMCIA -> USB adapter. To make sure this wasn't the culprit, I stuck the CF -> PCMCIA adapter into a laptop. Thankfully, it works fine. So I guess it's this POS usb thingy. Oh well. Thanks for the help Tony, but I think I'll be fine (no heart attack).

EDIT: The S400 formats the card as FAT16 in case you were keeping score