Turned out that accessing a FlashCard from a VirtualBox Debian running on OS X Lion is not so straight forward as first anticipated. Well, a little googling found the answers needed and here’s a setup that worked for me:

Plug in the card reader (or any USB device) to your computer and let Finder mount it.

In VirtualBox select Settings -> Ports -> USB. That should you present with a dialog where you can add and edit Filters.

Click the green plus sign and in the then opening dialog delete everything except Name, Vendor ID and Product ID.

Now start your VirtualBox Debian and check dmesg. The reader should have been recognised as /dev/sda.