In one embodiment of split data migration in accordance with the present description, to migrate the data of a volume or other data unit which has been previously mirrored from a primary data storage system to a secondary data storage system, the data unit is split into two or more parts. One part is migrated from the primary data storage system to a migration target storage system, and another part is migrated from the secondary data storage system, to the same, common migration target storage system. In this manner, the migration workload of migrating a unit of data may be split between the primary and secondary storage systems, reducing the impact of migration data processing upon the primary storage system and reducing migration time. Other features and aspects may be realized, depending upon the particular application.