Uncle B, did you format the player (FAT16) from your Windows box? All sorts of bizarre behaviour can occur if you don't format after converting to the UMS firmware (well, all sorts of weird crap happened for me on my linux box, so I'm asuming that it might for windows too). Please forgive me if I've given you a useless answer
What about if you just transfer files via Windows explorer, do you still get the "ERROR WRITING FILE"? If not, it might indicate the WMP and your iFP aren't playing together nicely. Again, just a guess, since I don't use WMP (or windows for that matter).