Looking at the example for the MassStorage class it look like you cannot have the file system mounted at the same time as having the MassStorage attaced to the SD card (PersistantStorage class).
When I try it I get an exception when calling EnableLun. I would suspect that even calling AttachLun when you have the SD file system mounted might not be a good idea.
What is the reason for this restriction? I don’t think it is a problem really, just curious.