To set HDMI output for Recalbox External HD setup, connect your laptop to the TV using an HDMI cable. Ensure the TV is powered on and set to the correct HDMI input. Boot up your laptop with the Recalbox External HD connected. Once booted, access the Recalbox settings menu, usually found in the main menu or through hotkeys.
In the settings, navigate to the display or video options, and select HDMI as the output source. Adjust any additional display settings as needed. Save the changes and restart Recalbox. The video output should now be directed to your TV through HDMI, providing a more comfortable gaming experience.