I have Asus X200MA Laptop with UEFI Firmware & Windows 8.1 pre-installed in UEFI mode (CSM Disabled). Now if I turn CSM on, I find that it still successfully boots into Windows 8.1 which otherwise uses GPT partition scheme.
I am curious to know how is this possible? if I correctly understand, CSM emulates BIOS Mode booting in the firmware. So it should look for MBR disk and since it does not find any should not allow booting Windows 8.1 off GPT.