Nothing to add except to reinforce what Chrispy said. I'd bet that's the source of your problems. As he said, look on ebay for a Gigabyte, Asus, or AsRock mobo. IIRC, MSI was having some issues that generation.
There are a few threads on TR talking about OEM computers not supporting newer UEFI graphics cards. Seeing your Medion mobo suggests you've got an OEM computer even though you never explicitly stated it.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod