Close all applications. Open the file manager (Utilities -> File Manager). You will see top level directories for "Nokia N810" (the Device memory), the "Internal memory card" and the "Removable memory card". Use the stylus to drag and drop the "map" subdirectory from the "Internal memory card" to the "Removable memory card". It will take a while, but when it is done you can open the "Map" application and it will find your maps properly.