DELL 8300 XP Pro Hard Drive ???

 I know it is no longer supported but it is still a good gaming machine the wife n kids can play on with no hassles. My problem I posted before, probably in the wrong spot, is that the 80GB hard drive with the OS on it reads letter D: and my lower 500GB drive reads C:. Now I tried to eliminate it by just removing the 500 drive and the computer will not start up then. I have to have both in for it to boot windows and the frustrating part is it takes so long to boot up. I tried putting the hard drive in the opposite slot to no avail. I have used the ribbon properly at the end so what am I missing. HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! TY!