Abstract: The present invention is a controller and a method for its use in a disk data storage and retrieval system. The controller is of the type which associates a file mask with a sequence of commands. The controller has means for executing a critical-section-start (CSS) command received from the host computer which indicates to the controller that a critical sequence of commands will follow and places the controller in a critical sequence mode (CS mode). The controller has means for executing a critical-section-end (CSE) command which indicates the end of the commands. While in the CS mode the controller checks the file mask for each command received and refuses to execute selected commands which are not associated with predetermined values of the file mask (CS file masks). A program running on the host computer sends the controller a command establishing a CS file mask and the CSS command, then the program sends one or more sequences of critical commands to the controller.