A PRECOPY command identifies source and destination devices. Data begins to transfer from the source device to the destination device in a background mode under the control of a copy program that transfers the data on a track-by-track basis. Changes to the data tracks are monitored in three lists. The copy program monitors the first two lists to determine when a data track in a source device has been written and needs to be retransferred to the destination device to provide the most up to date data. Procedures assure that any data access to a particular track in either the source by any application prior to the activation of a formal transfer are accommodated to maintain data integrity. An ACTIVATE command makes the destination device available to another application and activates another copy operation. Procedures assure that any data access to a particular track in either of the source or destination devices by any application prior to the transfer of that track are accommodated to maintain data integrity.