You could try a really useful utility for formatting SDCards. You will loose any data on the card, but it should then be fully functional.

http://www.sdcard.org/consumers/formatter/

Make sure you download, read and understand the manual as well.

I also saw (but haven't tried) this: http://panasonic.jp/support/global/cs/sd/download/sd_formatter.html

Woggle, February 2010