I just formatted my HDD from Windows 10 so I can put Windows 7 on it. Although the drive is formatted, it stil somehow boots to the Windows 10 repair menu. Putting in the Windows 7 flashdrive doesn't do anything. It totally ignores it and goes straight to Windows 10 repair. Even setting the Flash Drive as number 1 boot device doesn't fix it. I've had problems with the HDD before, namely that the MBR was damaged and I had to fix it during Windows 10 installation process.