Maybe one of these links will help.
http://www.rmprepusb.com/tutorials/54—how-to-fix-write-protected-disks
http://forums.sandisk.com/t5/All-SanDisk-USB-Flash-Drives/HOW-TO-REMOVE-WRITE-PROTECTION-FROM-USB-FLASH-DRIVE/td-p/283430/page/5 8/7/2013 entry