Our W211 estate has recently started to have an intermittent problem with the COMAND sat nav. The main screen pops up a message saying "Can't read DVD". The the navigation voice instructions continue to work and the display in the instruments is still working. The Can't Read message stays up on the main screen for some time but then the maps come back briefly only to disappear again. I have removed the disc in the rear to check for dust etc but it seems to be clean and undamaged. Ideas please.