Well, I installed Comix on Nokia N900, and in File->Open, it uses almost stock file-dialogue. In it, I see directory /home/user:
Nokia N900
Xorg.0
et cetera
I cannot navigate to root directory ("/") (I can navigate to "/home", and not higher). I don't have a memory card which I could insert into the device. Maybe, you could navigate to "/home/user" directory and see whether there is an-external-memory-card alongside with Nokia N900?
Best wishes. Bumping the thread...