Answering my own question here, but it may be of benefit to other readers who experience the same issue. In this case, again Google has been a good friend...
The 2012 nüvi models are treated as MTP (Media Transfer Protocol) devices by Windows Vista and later and will not mount as lettered drives like the older nuvi's. To change this behaviour, carry out the following steps:
1. Power up the nüvi and go to the map view screen.
2. Select the speedometer screen.
3. Hold the center of the speedometer for about 10 seconds. This will open up the Diagnostics Page.
4. Scroll down to MTP Settings and select.
5. Change setting from MTP Auto Detect to Mass Storage.
6. Click Save.
6. Switch off the nüvi and connect it to the computer by USB cable.
7. The unit will now be recognised as mass storage device and all files and folders will be visible. *)
*) In Folder Settings of the computer used, enable to show hidden files and folders as well as hidden operating system files!