I have the same problem. Same, it is a Sandisk Cruzer USB. It happened suddenly, 1 day I decided to format and hope , enable to write, or write protected.
During my research peoples, I read that it happen after using something like secure vault, or dont know what. but I never used that $@$* secure vault or dont know what provided by Sandisk.
Another reseach told me that those Cruzer USB are faulty and should be sended back, they gona provide another one.
Except warranty is gone + in my country no warranty disponible or Sandisk store.
I tried everything, even GParted through DOS. Nothing works!!
Never ever buy again a Sandisk or Cruzer product!! Luckily there is no data on it.
My best guess, internal software need to be flashed back, with something through DOS, not through Windows, cause Windows is very poor for managing other devices than itself. Maybe by Linux. Unfortunatly I could not find anything over the net for this problem, most people say it is locked by the small Switch present on the USB, except there is no switch!