I would suggest that you do what John has suggested. If what he suspects is in fact what happened you should just live with the change. This shouldn't cause any issues with the operating system. Since you are aware that the D: drive is now the F: drive you could just leave those letters as they are.
If you post a screenshot of the Disk Management we will be able to confirm if the F: and G: are both Windows RE partitions.
If you need instructions to make a screen shot and how to post it in your topic I've provided this below.
How to make a screenshot and post it in your topic
1. Download and run A Thousand Words .
2. Follow the wizard to capture a screenshot.
3. Use the built-in editor to resize, edit, or re-capture your screen shot.
4. Your screen shot will be saved to your desktop when you finish the wizard.
You can post the screenshot in your next post as an attachment.
Just below the area where you write text in a post there is the Post button, to the right of this is More Reply Options.
When you click on More Relpy Options you will see Attach Files and Browse, click on Browse, this will open Pictures on your computer, click on the image you want to post, then click on Attach This File, then Add Reply.