If you want to add a 2nd hard drive, just plug it in & Windows will auto-assign a drive letter to it. Then, if you really want to, you can assign a different letter to the DVD-RW drive to free up D: drive.
Windows has a set way of assigning drive letters.
For the most part a hard drive will take priority over a removable device, or one with removable media inserted in to it.
By adding another hard drive or creating a partition of an original drive C:
The new drive or partition should move the drive letter of the removable media device down it`s letter chain of media devices.
HD C: D: CD drive E: F:
You can use windows disc manager of windows, by typing : Disk Man
Right clicking on a drive or device and change drive letter.
As long as it is not in use by another device in the letter chain. The result of having two drive letters the same will render one unseen by windows, or like it has, disappeared, or is not functioning.
That is a letter or drive letter assignment clash.
Example: two C: devices cannot exist.
Only one will show.
So Disk man must be used to allow the device to be presented. A-Z letter device letter chain one for each device on the system.
Be aware that if any programs are installed or run off the drive, that by changing the drive letter it will no longer execute as the shortcut to the drive letter or path will no longer exist. This relates to hard drives internal. With programs installed on the drive set to run through windows via a shortcut on the desktop ect.