Thanks for the replies. I tried reformatting and using immediately in the n800 and it didn't work, unfortunately. As a last resort I downloaded sdformatter ( http://panasonic.jp/support/global/c...formatter.html ) which did format OK. I was able to copy the backed-up files onto it again and it was recognised by the n800.

It has worked since. However, last night I used the n800 to download a 300 mb file and this morning the card again is no longer recognised.

It looks like I'm going to have to replace the card with a new one. Can anyone please recommend one?