Have U3 Avast on 2 Sandisk thumb drives. Both are set to auto start. On one it does not; however, can manually start it.
Have unchecked auto start, closed program, and opened and rechecked, but nothing.
Could it possibly relate to Roboform also auto starting on this one drive? Can’t find a method to turn off RF’s auto start to test it. Or could a file be corrupt; and if so, how to reinstall?
Sorry, found the answer in Help, and it now operates on startup. Had to go into the U3 Launchpad, Manage U3 Programs and check it there to start. Interesting that I don’t believe this was necessary on the other thumb drive – but perhaps so and I’ve forgotten!!
Welcome to the forums, Miles. 
Glad you got it sorted out. Be sure to visit the forums often for more info and perhaps help others when you can.