http://www.nikonchat.com/nikon-memory-card-locked/

Nikon D40 help! please answer!?
That is "the memory card is locked. Detent slide to" write "position." What does this mean and how I can fix it??
There is a small (usually) white tab at the top left of the card (facing front) should be in the position to the top. Sometimes, when you insert the card into a card reader or the camera may inadvertently slide to the position blockade. Chris