Boot from the Windows 8 DVD and try restoring your system.
Since Windows 8 is three years newer than Windows 7, the drivers loaded at boot time should include those necessary to handle your USB3 external disk.
DISCLAIMER: I HAVE NOT TESTED THIS IDEA! I AM ONLY SPECULATING!
Cheers, PW.
P.S. Those who did not get a Windows DVD with their computer may wish to download a Windows 8 Evaluation .ISO and burn a disk. Even if Windows 8 is never tried, the disk still has the code needed to repair systems that are unable to boot.
And if they want Windows 7 (SP1) .ISO files, use these links. . .