Why does my Compact Flash GPS card work in a USB card reader?

I have a Compact Flash GPS card which works fine in my PDA. I thought that if I put it into a USB card reader (into the CF slot, obviously) and plugged the reader into my laptop, then Windows would prompt me for a driver and the rest would be simple. It doesn't work. Windows does not prompt for a driver - in fact it doesn't seem to 'see' the GPS unit at all. Why not? Is there a way to use this card with my laptop? Thank you.