How to restore system image on my second partition

I upgraded to win 10 on my pc
bt I discovered my gaming is too slow,
so I want to install win 7 through a
system image I created on my second
pc( the same type and specs but
different username and o.s since I
upgraded my main pc from win7 to
win10) onto the new partition I
created on the pc ( for the win7
installation). The external hd I used
to create the image was western
digital hd bt its not showing in the
'select usb drive' when trying to
restore the image. I used windows backup for the image; I didn't use third party app.

Edit:: I actually want to dual boot
win10 and win7