Source: 

I did not have to do this IF Windows 7 was installed on any connected HDD.
But, with only one clean (factory new or equivalent) drive attached, Windows 8 Pro Upgrade would not activate after a clean install.

  1. Open regedit. Navigate to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/
  2. Change MediaBootInstall from 1 to 0
  3. Open Command Prompt (cmd - run as administrator).
  4. Type
    slmgr /rearm
    hit enter.
  5. Reboot Windows.
  6. Run the activation utility afterwards, enter your product key to activate Windows.
    Not always required - check status first. I had all UAC features disabled and this may have eliminated the need.