notified in writing at least thirty days before the hearing of the case by a board of inquiry of the reasons for which the officer is being required to show cause for retention;

allowed reasonable time, as determined by the board of inquiry under regulations of the Secretary, to prepare his defense;

allowed to appear in person and by counsel at proceedings before a board of inquiry; and

allowed full access to, and furnished copies of, records relevant to the case at all stages of the proceeding, except that a board shall withhold any records that the Secretary determines should be withheld in the interests of national security. In any case where any records are withheld under this clause, the officer whose case is under consideration shall, to the extent that the national security permits, be furnished a summary of the records so withheld.