Well I just got caught... The manual for my Lowrance Elite 4 DSI states
You can save screenshots to a microSD card by inserting a microSD card into the card slot and pressing Power and then Zoom (+).
Er, well, yes. I bought a blank micro SD card, with the intention of capturing screenshots of the trails I save, so they can then be deleted to make room for more. Sounds logical doesn't it...
Well no. The moment I removed the Navionics Silver card from the micro SD slot, the screen went blank, and the "blank" map of Australia appeared.
I rang a local big Lowrance/Navionics dealers who confirmed that yes, you lose your charts when you swap cards, so you CANNOT make a screenshot of your trails. Presumably the ONLY screenshots you can therefore ever get are of the simulator images, like those in the manual.
Thank goodness I only wasted $3.99 on the blank micro SD card...