Why is my external monitor not working?

If your Mac has an external monitor connected with a 4K or higher resolution, and you notice Luna's connection is not stable or has dropped in resolution, try disconnecting the monitor to see if there is any difference.

On some Macs, like a Macbook Pro, Luna may not be able to work when the Luna hardware is plugged in on the same side as a monitor with a 4K or higher resolution. This is because of how the Macbook's hardware is laid out. We recommend plugging Luna into a port on the opposite side of the Macbook from where the display is plugged in.

For example, a 2019 Macbook Pro has four Thunderbolt 3 ports (two on each side of the Macbook Pro). Each side of the Macbook Pro has one Thunderbolt bus controller, which is shared by both Thunderbolt 3 ports on that side.

Image source: Rocket Yard, the official OWC blog

If you have a Thunderbolt or USB device connected to the same Thunderbolt bus controller as Luna, there is a chance that the Thunderbolt or USB device will use most or all of the bandwidth from the Thunderbolt bus controller, and not leave enough bandwidth for Luna. This is common with Thunderbolt docks or hubs, or external monitors with resolutions of 4K and higher.

If you have a newer M1 Mac with Apple Silicon, there are some limitations on the number of external displays allowed. For example, the M1 Mac mini supports only one display using Thunderbolt/USB-C. Be sure to check your device's technical specifications to see what is compatible with your machine.