Well I'm not an expert but I was experiencing the same problem at one point in time trying to do a clean install of Windows 7 from an install DVD. I too have a Gigabyte motherboard with Dual BIOS/UEFI. I found that the solution was to press F12 on bootup - so that you get the screen that allows you to explicitly choose what device you want to boot from. I chose to boot directly from my DVD drive, where the install DVD was, and that solved the problem - I was then able to install to MBR partitions. Note that the difference here is that I was doing the install from a DVD, not a USB flash drive. So maybe if you try installing from a DVD and doing the above it might solve your problem. (But who knows, maybe it you had tried F12 and explicitly choosen to boot from the USB flash drive that might have worked too. Worth a couple of minutes of experiment to find out. And if you find that it does work, of course, post the result of your experiment up here - it might help others in the future. )