It is really meant as a program to migrate an operating system.
It can be used to backup as well.
Since they are giving it away - too good an opportunity to miss.
They do free one that is intended as an image backup program only.
You might find that easier to use. Free Backup Software: Paragon Backup & Recovery Free Edition - Overview
The idea is to make an image of your operating system - with a 3rd party program like paragon - you can store that image anywhere - e.g. on an external drive.
You can also restore it from an external drive.
You can't do that with windows system image program.
Use a 3rd party app. instead for the extra flexibility.
Then you can delete all the stuff that windows is putting on your D drive.