HDD not recognized GPT UEFI

Hello, I've got an Asrock Fatal1tyZ68 Professional Gen3 Mainboard. Before I had Windows 8.1 installed on a small 512GB HDD with MBR and everything worked fine. Now I've switched to a 6TB HDD. The Problem is I had to switch to GPT because of the size. The strange thing now is. When I unplug and replug the HDD it is recognized as a bootable UEFI option. When I restart it is not recognized as...