I have a Windows 8 HP that the MoBo is fried. While I wait for a new MoBo to arrive I wanted to move my Windows 8 HD to my old Windows 7 computer. When I install the Windows 8 HD into the old machine I get a blinking cursor after the BIOS start up.
I do not think windows 8 will allow you to do that (it is also against the EULA) OEM licenses are imbedded into the BIOS/UEFI windows may be trying to validate hardware before starting, but you can also check your boot order and ensure the computer is trying to boot from the disk you want it to. if HP is sending you the MB ensure they also send recovery media because you may run into this problem when you install the new MB.