I made them disappear through the safely remove hardware thingo. I know if you 'stop' either the name of the memory card (e.g. Lexar media memory card) or the drive of the slot that holds the card (e.g. G:\) in the safely remove dialogue box, it removes all of the drives altogether. I always forget which one it is that removes all drives and which removes just the drive that is holding the card you are using.
I have a USB card reader on another computer that I usually just unplug and plug back in to get the drives to show up again, but this one is permanently in the monitor so I can't do that. Halp?