A method of operating a data control system to copy a volume of data from a primary storage system to a secondary storage system comprises receiving an instruction to copy the volume of data, wherein the volume of data comprises a plurality of data blocks, in response to the instruction, determining if at least a data block of the plurality of data blocks requires copying to the secondary storage system, if the data block requires copying to the secondary storage system, then reading the data block from memory in the primary storage system and transferring the data block for writing to memory in the secondary storage system, and if the data block does not require copying, then transferring a synthetic block for writing to the memory in the secondary storage system.