Hi, I had the same problem and tried everything suggested but nothing worked.
Finally I found the answer was to use a Flash drive tester - in Destructive write test mode. You will lose any data not saved but you can now re-format the drive. This is the link to a free program,
http://mikelab.kiev.ua/index_en.php?page=PROGRAMS/chkflsh_en
Hope this helps,
Graham.
