Within one day I somehow "replaced" my old, defect Z68x-UD3H with the Z77X-UD3H and I can now reproducibly start from just one 2,5"" HDD with macOS Sierra (10.12.3), which has Clover 3766 on it (personality is a iMac13,2 - i3 3,30 GHz, currently 8GB RAM, BIOS F17).
As was recommended here above...