Something similar happened to me to and I just reformatted the card.

Can't remember which menu it was in as I don't have my GT31 with me.

I go with mineral here, other people have had a similar problem because the card itself was "locked". it's just below the contacts, on the straight side, not the side with the corner cut off. And it should be in the position closest to the contacts.

TheSailingMoose said..
The card was locked but now i am getting a 'bad format' error. I know how to change the format but what do i need to format it to?

Its been a while since I've done it but I think the GPS can format the card. You could always format the card in a computer first to clear it up. Do a full format perhaps but I think you can only do the format in FAT32 on a computer unless you know what you are doing while the GPS probably wants a FAT16 format. This is all speculation on recollection on my part so could be completely wrong.