Guys, disabled the Internal Graphics in BIOS and all 8GB are usable now.
Going off-topic here as the prob seems to be solved. I have no idea about this but do desktop pcs have anything similar to Nvidia optimus? If so, that internal graphics might save a lot of power.
I believe some motherboards offer a feature that can come in handy. The name varies from vendor to vendor. They will activate the video ports on the motherboard and enable a monitor to be run using the iGPU.
Not sure regarding switching to iGPU when on the Windows desktop, but about the last three generations of GPUs have their own low - power modes. My GPUs sometimes clock all the way down to 20Mhz - though usually 'idle' at 80Mhz.