

Not every external drive is optimized for Macs. Restarting your Mac essentially clears your macOS’s memory and starts it up fresh. Or press and hold the power button and, when a dialog box appears, click the Restart or press R. Hopefully, some data damage that can be fixed by restarting. Sometimes, if a USB disk won't boot, the cause is macOS issue. If you’ve got another port, try connecting the device to that one.

It could be a hardware issue with the Mac. Try connecting the same device with a different cable. Assuming it’s plugged in correctly, not wobbly or loose, the cable could be at fault. It sounds obvious, but since this relies on a wire - either a USB cable or HDMI cable - if it’s not connected properly then it won’t appear on your desktop. Check whether the drive is properly plugged in.
