[Ignore this post; I was wrong; see Post #8 below by Ace. I just ASSumed that this was a possible problem. After reading Ace's post I tested it out, and I had no trouble writing to the supposedly locked card.]

I expect this is not your problem, but I'll put it here should anyone else bump into this thread whom it might have helped ...

Also make sure you didn't accidentally push the little slide mechanism on the edge of the card into the locked position.