Feb 24 (Reuters) - A sixth member of the board of the Electric Reliability Council of Texas (ERCOT), the state’s power grid operator, resigned in the wake of a massive failure of the state’s power network. Randal Miller, who represented independent retail power providers, resigned late Tuesday, board Chairman Sally Tanberg said on Wednesday. The disclosure began a the first meeting by directors since outages that left millions without power, heat and water for days last week. (Reporting by Gary McWilliams Editing by Chizu Nomiyama)