Hi Toots,

The fact that 1 screen is running off the R9 280 tells me the system is ok and the grahics card as well. 
In the Bios of the mainboard you can select the option to have the system initialize on the PCI-E Graphics or the Integrated Graphics. Normally this will be set to “Auto”, i prefer to disable the integrated graphics completely or chose for the option “PCI-E Graphics only” which normally doe sthe same thing. This means the Graphic outputs of the Mainboard dont function anymore and you should use The R9 280. 

Please check if the DVI-Output-Connector on the graphics card is ok. In this female connector are little gold plated connection pins that sometimes get damaged when attempting to force in the DVI plug the wrong way (or something like that). Please check if the DVI Cable is OK and last but not least, please check if the Screen or DVI input if the Screen is ok.

Okay, this is what I did:

I connected both screens to the graphics card, one with HDMI the other with DVI and restarted the computer. During starting up I enabled UEFI (BIOS Asrock H81M) and both screens worked fine (both showed exactly the same). Graphics adapter was already set to ‘PCI Express’.
Then restarted computer again and when Windows 10 was starting up both screens showed the blue windows flag and the circling dots for 5s or so, then the screen with the DVI cable went black and showed ‘no input signal’ and Windows appeared only on the HDMI screen.

Plugging in the Sub-D cable from the mainboard to the ‘DVI’-screen after disconnecting the DVI-cable from the graphics card, made the screen work again, so controlled from the mainboard, without changing settings in the UEFI.

Someone any clue?

Hi Toots,
When the system recognizes a discreet Graphics Card the BIOS of the mainboard should disable the on/board Graphics. Please check for BIOS update for the mainboard because this does not sound right.