My PC has two hard drives, on one I have updated to Windows 10 successfully, on the other which is running Windows 7 Ultimate successfully I can't upgrade to Windows 10. I always get the message this machine is not compatible. It clearly is as it is already running it after upgrading the first drive. Is it because in the Registry, Windows 10 has already been downloaded and in the HKEY_ /OSUpgrade entry a date stamp has been entered, so do i have to enter a new DWORD "AllowOSUpgrade and set a value of 0x00000001. A copy of registry entry attached.