Source: pchelpforum.com

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.