I'm on 10.10 and I suspected that my solution was unlikely to be the solution for others, which was the reason for not posting it in the first place. Finally I decided to post it anyway since it may help someone. Checking the hardware may be the first thing to do, but I was completely misled by the behaviour of the scanner working flawlessly in Ubuntu after having run Windows. Obviously, the USB communication in Ubuntu was good enough for the scanning process but not for loading scanner firmware. Difficult to say if Franko30 or Grafen even have the same problem, i.e. that it is the firmware upload that fails. At least ndaneau seems to have the firmware upload issue, would be interesting to hear of any progress from you?