Because your D drive is disk0 according to the bios, windows would have put the boot files on it when installing. I know, it says C: is the "boot", but that's
Microsoft logic.
Here's an official support article about restoring boot files:
http://support.microsoft.com/kb/2004518You'd want to switch the order the hard drives are physically connected within the computer first otherwise your computer will always be booting off the boot files on disk0 if they are present before moving onto checking other drives.