Thanks to you both - I'd seen the tutorial . The card is a proprietary Garmin Data card, not the generic type SD card. The GPS units that used this card were the likes of the 276C, 478 and 172 chart plotter, Garmin Part Number: 010-10226-21. The GPS units were not touch screen and the diagnostic mode (on a 276C Press and hold "Enter" and while doing that press and hold the power button) brings up a single page of text but the Data Card Is "Unknown" - even though the GPS software is the latest and the map is a open source map.

MapSource will only send a map to the card programmer but will not show the Unit ID or the files on the card. BaseCamp won't even see the card programmer.

I suspect, because it's a Garmin card plugged into a Garmin card reader, that the ability to work with data on the card has been severely crippled at the design stage.

As you say, the Fast Unlocker may be the only answer.