Thanks for the information and I managed to get the nuvi 1690 in to preeboot mode for a short time directly after power up. It will not stay in preboot mode for long but maybe enough time to use tools to repair it.

Do you know where I can get hold of the NA firmware to try and edit the HWID version? Garmin refuses to let me download the nuvi 1690 EU firmware directly and only points out that I need to use the Garmin Express to do that but as it is not seen as a memory device in preeboot mode that will not work.