From site/manual Kenwood:

8) If you receive the message "DISC ERROR" means that the unit has attempted to read the disc for playback. In this case you need to go to the navigation screen and follow the prompts on the screen. DO NOT TURN OFF THE APPLIANCE during the upgrade
DVD with folder garmin\gupdate.gcd (comply with the syntax)

You may also try the WEB UPDATER with the PC and the USB cable back

Bye,bye