Can anyone help with this problem (Audi allroad 2005)?

My sat nav is built-in via cd. The sat nav cd can be removed and the unit act as a single cd music player if required, car also has seperate 6 cd changer controlled by same unit. Recently, sat nav has stopped working, keeps asking for sat nav cd to be loaded although its already in machine. When the sat nav cd is removed and re-loaded machine advises cd is being checked but then asks for the sat nav cd to be loaded. After ejecting and re-loading machine advises cd is being initialised and then asks for the cd to be loaded, so I remove it and re-load it and machine advises its being checked and then asks for it to be loaded again. The whole process of checking, initialising and asking for cd to be loaded keeps on and on. I have purchased a new disc from Ebay, cheaper than any where else, but same problem. When used as a music cd player it works perfectly, as does the radio etc. Any ideas