Most SD cards have a switch on the edge of one side of the card. Sliding it up or down will unlock or lock the card. When the card is locked, you will not be able to transfer any audio tracks. Nor will you be able to play any copy protected files. We recommend you always keep the card unlocked.
NOTE: We have found that on some cards, if the slider is even halfway towards the lock position, the card will be locked. Make sure that the slider is all the way to the UN-locked position.