Check the device: Make sure the device is turned on and its cables are connected. If the device requires a power cord, make sure it’s plugged in. If the device is in a chain of devices, remove the device from the chain, then plug it directly into your computer and verify that it works.

Check that you’re using the right cable: Make sure you connect the Thunderbolt device with a Thunderbolt cable, not a Mini DisplayPort cable. A Thunderbolt cable has a Thunderbolt icon on the connector.

Check that you’re using the right port: Make sure you connect the Thunderbolt cable to a Thunderbolt port, not a USB-C port. See the Apple Support article Identify the ports on your Mac.

Check for device firmware or software updates: Make sure you installed any software and firmware that came with the device. Check the manufacturer’s website for the latest software.

Check chained devices: If the device is in a chain of connected Thunderbolt devices, each connected to another with only one connected directly to the Thunderbolt port, disconnect the device from the chain and verify that it works (see “Check the device,” above). Then make sure that all the devices other than the one connected directly to your Mac have power cords. (If the device connected directly to your Mac requires a power cord, it should have one, too.)

If a Mini DisplayPort device is part of the chain, make sure it’s at the end and is connected with a Mini DisplayPort cable.

Verify that the device appears in System Information: On your Mac, press and hold the Option key, then choose Apple menu > System Information.

In the window that appears, see if the Thunderbolt device is listed below Hardware in the list on the left. If the device appears but does not work, review the device’s documentation for further troubleshooting steps.

Reopen apps: Quit and reopen any apps that use the device.

Restart your Mac: Choose Apple menu > Restart.