Re: is Windows8 compatible with ASUS P5G-MX motherboard or not
The USB 3.0 thing is probably not relevant for your board. But perhaps some other driver is. Since you have used both DVD and Flash drives, it would seem one of those would have drivers in Windows 8 for your devices...just not sure. Have you checked for any Bios updates for the board?
You can install Windows 8 upgrade, for testing, without a prior OS...it just won't activate.
If you are installing to a separate partition, you probably don't have to uninstall anything. I installed to a clean hard drive with the sacrificial OS on another drive.
If you are using the Download for the upgrade, let it make the flash drive, or DVD.
Computer Type Homebuilt Asus P8Z68-v Pro UEFI install
CPU Type and Speed i7 2600K