n StarWind Management Console, select the device that you want to extend and click the Extending Size of HA (High availability) Device/Image File Size button in the right side window. Then, specify the disk space that you want to add and click the Finish button.
Link to KB article: https://knowledgebase.starwindsoftware.com/maintenance/how-to-extend-image-file-or-high-availability-device/