Using PC linux with the n900 connected via mass storage typing dmesg on the pc will produce alot of crap. Down the bottom their should be some messages about device sizes. You should be able to id the sd* which corresponds to you microSD. If your N900 is connected you should see it in this area. Another way is type sudo gparted and go through the drop box on the top right and find your microSD card (using the capacity) and the drop box should have the sd* of the device. I did this on ubuntu 9.04