Restore Windows 10 system image via Boot Camp?

I've been having some trouble with restoring a Windows 10 system image on a Boot Camp partition.

I installed Windows 10 via Boot Camp some months ago when my main OS was Sierra. After the small partition space filled up quickly, I decided to save my complete instance of Windows 10 through a backup, and a system image (by which time I had already upgraded to High Sierra). These files were placed on an external hard drive.

I removed my Windows 10 partition via Boot Camp Assistant, and did a clean reinstall with a larger partition. From this point, I realized I needed to use the Windows Media Creation to boot the restore from, so I used a USB flash drive for this purpose.

Now, I boot my MacBook (while holding opt/alt), select the EFI USB flash drive to boot from, go to the system image recovery, load my external hard drive, and it even shows the backed up system image, however I'm getting a message that says:

"To restore this computer, Windows needs to format the drive that the Windows Recovery Environment is currently running on. To continue with the restore, shut down this computer and boot it from a Windows installation disc or a system repair disc and then try the restore again. If you don't have a system repair disc, you can create one now."

I thought I booted the computer via the restore flash drive?

Any help would be greatly appreciated.