I'm not really sure how a map card goes bad...

You said the card slot on the Status screen says nga-10... And the nga-10 switches to card slot 2 when you changed slots....so the unit is recognizing the card...

Try this...
Take the card out of the unit...
Turn unit off...
Repower unit on into "Normal" mode...
Open any "Chart View"...
Insert card..
There should be a small white window "popup" at the top of the screen that says "Loading Charts"...
If you get no "popup"... Sounds like the card went bad..

How old is the card..??
You might contact Navionics and see if they have any suggestions..

Rickie