Open.HD automatically stores video and telemetry in a temporary storage while it is being received from the Air SBC.
To save video, telemetry and screenshots to a USB memory stick, simply plug the USB drive into the Ground SBC AFTER you are done flying. After a few seconds, a message should appear and the recorded video will start playing on the HDMI screen while being saved to USB. A message will appear when it is done saving.
These are the relevant settings: