I had to flash the latest BIOS into a Gigabyte GA-P55-UD6, because the system didn’t recognize some more modern devices on USB. All settings are more or less standard (no overclocking, etc.). Only SATA drives are mounted, no RAID. After this I checked and updated CMOS settings and set boot order. Nevertheless when booting the system hangs with ‘Loading Operating System ...’ unless I go into boot manager (F12) and select the correct hard drive.
Obviously I've missed something, but what?