Sometimes when you get the write protected message for SD cards or USB flash drives it might mean that they've reached the end of their life. Or in other words - it might have failed.
However, there are still some thing you could try in order to confirm that, like trying to reformat it via a different device or the manufacturer's formatting tool. Or try the suggestions from this article: http://www.wikihow.com/Disable-Write-Protection.
Hope that helps. Please let me know how everything goes.