1. Data migration apparatus for use in a data processing system including an existing storage device for data elements connected to a host system that performs data element transfers with the existing storage device in response to data transfer requests and a replacement storage device, said data migration apparatus comprising: A) connection means for forming the existing and replacement storage devices into a composite storage device with a path to the host system thereby enabling the composite storage device to respond to data transfer requests from the host system; and B) data migration means for migrating data from the existing storage device to the replacement storage device concurrently with data transfer requests from the host system.