Based on this topic it seems the bootloader is damaged. You may find more advice here:
http://forum.mikrotik.com/t/6-43rc5-bricked-rb1100ahx4/118941/1
Based on this topic it seems the bootloader is damaged. You may find more advice here:
http://forum.mikrotik.com/t/6-43rc5-bricked-rb1100ahx4/118941/1