Provided are a computer program product, system, and method for receiving, at a secondary storage controller, information on modified data from a primary storage controller to use to calculate parity data. The secondary storage controller receives from the primary storage controller difference data calculated from modified data and a pre-modified version of the modified data for a primary group of tracks at the primary storage and one of the modified data and new primary parity data calculated at the primary storage controller from the modified data and the difference data. The secondary storage controller uses the difference data and one of the modified data and the new primary parity data to write new secondary parity data and the modified data to a secondary group of tracks at the secondary storage.