Provided are a computer program product, system, and method for mirroring information on modified data from a primary storage controller to a secondary storage controller for the secondary storage controller to use to calculate parity data. New primary parity data is calculated from modified data for a primary group of tracks in the primary storage and difference data from the modified data and a pre-modified version of the modified data. The difference data and one of the modified data and the new primary parity data are sent to the secondary storage controller to cause the secondary storage controller to write new secondary parity data and the modified data to a secondary group of tracks at the secondary storage. The modified data and the new primary parity data are written to the primary group of tracks in the primary storage.