Are you plugging it in by itself or is the old drive still connected? Setting the drive letter is definitely not your problem, as that would happen when the OS loads. If you're BIOS isn't picking it up, you're looking at a hardware problem. Obviously the HDD works, since you already mounted it as a slave and copied everything over to it, which leads me to cables. Make sure the power cable is connected, and double-check your jumpers. My first reaction is that you misread the label telling you how to set it as master. If you're sure you have it set as master, try setting it to Cable Select, instead.