I updated the BIOS (to 4601 x64 9/8/2013), set it to Enable Multiple Monitors (I've got 4) and set Primary Graphics = PCIe (Radeon HD6670) and used the boot menu [F8]. There are 2 options for booting from the USB: UEFI and non-UEFI. UEFI errors, non-UEFI boots! I'm happy about that.