Well, that's the normal behavior of the board. If you open (on Windows) the device manager and go to the Disk drives section, while your Edison is connected, you should find a device called "Linux File-CD Gadget USB device". This device is in fact your Edison's massive storage drive. If you right-click on the "Linux File-CD Gadget USB device" and select Properties, you will find that in deed it is detected as a USB Mass Storage Device. This should be the same behavior on any OS.
Is this not happening to you? If so, there may be a software issue with your board. In that case I'd suggest you to reflash your board either with Flash Tool Lite (Intel® Edison Downloads) or flashall (How to Run the Flashall Script).