Got it working again!

When I last removed the miniSD adaptor from the N810 I also took out the microSD card, and when I put it all back together again I jammed the microSD card as firmly into the miniSD card as I could - bad idea!

Pushing the card hard into the adapter seems to prevent the adapter/card/N810 combination from working... to get it working again I pulled the microSD card out of the miniSD adapter and this time re-inserted it gently - I didn't push it in any futher once I felt the miniSD adapter "grip" the microSD card. Then I slipped the miniSD adapter into the N810 and clicked it home by using the stylus to push on the adapter and NOT the card (so that I didn't push the card any further into the adapter).

Works for me!