The Amiga should see the drive without software drivers in SYS:Expansion, or wherever. The autoboot ROM should load the driver into memory automatically as one of the very first steps of the boot process.
I'm guessing either you have a bad 4.15 ROM (not likely, but handy that you can easily burn another), or that the Games/Autoboot switch isn't actually connected inside, so it's defaulting to no autoboot.
What version of FaaastPrep are you using?
