A network server migrates dataset blocks from a source storage device to a target storage device while continuing to service user requests for the data stored in the dataset blocks. A control driver executing at the network server tracks the location of the dataset blocks during the data migration as being at either the source storage device or the target storage device. Upon receiving a user request for the data, the control driver uses the tracking information to determine whether the dataset blocks associated with the requested data have been migrated to the target storage device or are still stored at the source storage device. The control driver then retrieves the data associated with the requested dataset blocks from the determined source or target storage device.