Ive bought a 2009/58rg golf & it has a rns 310 with sat nav, Its asking for a sat nav disc/media when i press the NAV button, does this radio need a disc inserted all the time to enable the nav to work, I presume i cant play cd's when the sat nav disc is in ?, so is the only option the SD card for playing music?, Any help would be great including can i use ebay copy type discs? as e budget is rather tight?, Many thanks!.