The present invention provides an improved method and system of improving the efficiency, and ensuring the integrity, of a data transfer in a serverless backup, or third party copy, system having one or more physical storage devices. The present invention provides improvements to the processing of serverless copy, or EXTENDED COPY, commands, and transfers of data associated with such commands. These improvements increase the speed at which such commands are executed and completed, and increase the capabilities of copy managers in serverless backup systems. The improvements also make better use of the storage devices involved in the data backup process. The method broadly includes determining a desired manner of execution of segment descriptors, or instructions, as a function of components of those instructions. Other aspects of the invention allow for execution of data segments of any size, and validation of segment descriptors and target descriptors prior to data transfer.