Dang, you’ve done everything I can think of, apart from a reset (hold the on button down for 15-20 seconds or more)–but I doubt that would help.
Perhaps phone SanDisk up, possibly for a replacement? In the U.S.: 1-866-SANDISK (726-3475). And other SanDisk contact information: http://www.sandisk.com/sandisk-support/contact-us