I had that same problem once with a Gigabyte MB and one other that I can't remember the manufacturer of.
When connecting a SATA Optical drive to the SATA ports, I found that sometimes they won't be 'bootable' plugged into the 1st set of ports on the M/B if those ports are set as AHCI. If possible, move the optical drive to a set of SATA ports you can change to IDE.