Before the governor shall make an order placing an officer on the retired list on the ground that such officer is disabled, unfit, or incompetent, the governor shall appoint a board of not less than five commissioned officers, one of whom must be a surgeon, to investigate and report on the matter. Such board shall determine the facts as to the nature and cause of the incapacity of the officer who appears to be disabled, unfit, or incompetent from any cause to perform military    service, and whose case shall be referred to it. Before entering upon the discharge of a member’s duties, each member of the board must be sworn to an honest and impartial performance of the person‘s duties as a member of the board. No officer whose grade or promotion would be affected by the decision of such board in any case that may come before it may participate in the examination or decision in such case. The board has the powers of courts of inquiry and courts-martial, and whenever it finds an officer incapacitated for active service, it shall report such fact to the governor, stating the cause of incapacity, whether from disability, unfitness, or incompetency. If the governor approves the finding of the board, the officer must be placed upon the retired list as provided in this chapter. No officer may be placed upon the retired list by the action of such board without having had a full and fair hearing before the board if, upon due notice, that officer demands it. It is not necessary to refer any case for the action of the board unless the officer designated to be placed upon the retired list, within twenty days after being notified that the officer will be so retired, serves on the adjutant general a notice in writing that the officer demands a hearing and examination before such board.