The onboard GPU is supposed to be disabled in the BIOS, not the Device Manager.
1. Boot PC
2. Press "DEL" key (probably, varies) at the right time to boot to BIOS
3. Change the BIOS setting for graphics so that PCIe (PCI-Express) comes BEFORE integrated graphics
*If the BIOS is set to IGP (motherboard graphics) you must leave your monitor hooked up to the motherboard or you won't see anything.
Once you exit the BIOS you need to shut down and reconnect the monitor to your graphics card (probably the DVI connection).
I'm really not certain why you're having problems. There is a new feature that called Virtu MVP (not just Virtua) that allows your onboard and addon card to work together but it isn't reliable.
Virtu MVP (If you have it) can be disabled in the BIOS.
**The other possibility is that you have an APU, not a CPU from AMD that has a combined CPU and GPU on the same processor. Unlike the Intel CPU's that can disable the added internal graphics, I think the APU's can't disable the graphics component. I'm not 100% certain but I thought you had two choices:
1) use ONLY the APU (CPU + GPU on same chip), or
2) use the APU + 1, 2 or 3 extra graphics cards (Crossfire)
Summary:
If you DO NOT have a APU then it should be simple. Change the boot order to PCIe first.
If you DO have an APU, it might be a problem getting the HD7850 working with it.