1. A method for backing up a primary logical unit within a data storage device, the primary logical unit and a backup logical unit together comprising a mirror-logical-unit pair, the method comprising: receiving a trigger I/O request by the data storage device; inserting a TRIGGER message corresponding to the I/O request into a queue that contains a portion of a sequenced stream of I/O requests directed to the primary logical unit; dequeing the TRIGGER message from the queue, and initiating a mirror split operation directed to the primary logical unit; sending the TRIGGER message to the backup logical unit; and after completion of the mirror split operation, maintaining the backup logical unit as a backup copy of the primary logical unit.