The built in Sat-Nav of my 2005 Allroad began to crash (usually at a critical point in my journey!) and it reported "Disk cannot be read" . My first reaction was to buy a new DVD which did not cure the problem .
Fortunately the guy from whom I bought the DVD on EBay was good enough to advise me to try a lens cleaning disc for the DVD reader. This I bought very cheaply and it did the trick. Since running the special disk my Sat-nav has been fine.

I was at first reluctant to believe that this would work because my DVD player was behaving fine with music discs but from this experience I have learned that the reading of a Sat-Nav data disk is far more critical than a music one

The advice I would offer is this - If your DVD type Sat-Nav misbehaves then do NOT take it to the main stealers and be told that you need to have a new reader fitted at huge cost. First buy a cleaning disc and give it a try.