If you use cable select (cs) setting on your hard disk, it will go to the first connection which will be primary; this of course depends on your motherboard. It doesn't matter if it is primary or slave, windows will assign it the next available letter in your windows explorer. If you want it to be D, if your other drive is not partitioned, once you have plugged in your new hard drive, unplug your cdrom(s) and start windows. Use your administrative tools to assign the drive letter you want and when this is done, shut down your computer, unplug the power, replug your cdrom(s) and windows will automatically assign the next available drive letter(s)