Welcome to the forum. I have to admit I laughed when I saw your thread title, you can't format/reformat a DVD Burner. So you can check that off of your list at least.
I would just remove the usb drive, right click on your DVD Burner in device manager and select to 'remove' the drive, and restart your computer. The computer should recognize the drive and hopefully reassign the drive letter D: for you on the restart. Not sure why the USB drive 'stole' the drive letter D: in the process. If it does it again, you can reassign it a different drive letter.