If you open Spybot start center,checkmark advanced mode,click Settings,then System Integration,there is Autoplay (current user),or Autoplay (global),which adds Spybot file scan to actions offered when connecting external media,but it's not intended to make your USB Flash drives unaccessible.I don't see anything else within Spybot that should affect that,either.I searched the forum,but that doesn't seem to be a known bug.
That's not to say that it's not Spybot causing it,just that I can't find anything that should affect that.
Is there any chance it could have been a coincidence?