Here are the facts. If you add a second physical hard drive, windows will, in all cases, label it as D: drive. You don't get a choice in the matter.
If you for instance have a main drive split into more than one drive such as; C: D: E: F: & G:. When you add that new second drive, your old drive will have partitions labled C: E: F: G: & H: and that new drive will automatically be labled by windows as drive D: Not sure why that is true, but it seems to be.