Yes, LambdaCalculus379 is right - you can't use microSDs on the Sansa c200 series with Rockbox yet (it is still a quite new port).
I know that the database struggles when there are no music files, that would explain the original poster's problem when all the music is on the microSD and can't be seen by Rockbox yet.
P.S.: I just filled out the DeviceChart in the wiki with some c200 specific info, hope it helps answering this kind of questions.