If you’re connecting your Mac to an external monitor using a USB-C cable. You should ensure the connectors on both your Mac and monitor are completely inserted. You may not be able to recognize your monitor even if there is a slight disconnection.
If you are using adapters to connect monitors that don’t use USB-C, make sure they are securely connected to one another and your mac. There are particular issues with HDMI to USB-C adapters that can prevent Macs from recognizing external monitors. Make sure your USB-C cable is connected to a DVI adapter. Furthermore, HDMI cables for monitors will almost certainly cause your Mac not to recognize HDMI external displays and are a deal breaker for the Mac.
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