A method of generating a backup copy of a database system. Modifications made to the database since the generation of a prior backup copy are summarized in a bit map on a page basis. When the next backup copy is made, only modified pages are transmitted and merged with the prior copy. Plural backup passes are made. New modifications are allowed to occur to the database on all but the last pass. Any database modification made at an address that has already been examined during a pass is backed-up during the next pass. Modifications are locked out at the beginning of the last pass to allow the final generation of a consistent and complete backup copy.