Perhaps you have updated your Taranis to a new firmware without having the "mass storage" de-selected? It is mentioned in the Taranis setup tips document:
https://drive.google.com/file/d/0BwSDHIR7yDwSOGhPMTFyWEpDVWM/view
Also, if you have installed Smartpropoplus and/or vJoy before you need to uninstall that software, it can cause a conflict when connecting via USB. It is mentioned in the manual:
https://drive.google.com/file/d/0BwSDHIR7yDwSelpqMlhaSzZOa1k/view?usp=sharing