There are many reasons that may cause your external hard drive not to show up on your Mac, for instance, the USB cable is broken, the external hard disk is formatted in an incompatible manner, etc.
Here are four methods for you to fix the external hard drive not showing up on Mac:
And then, when you type in the command you mentioned, the Terminal says "unable to find disk", that's may because your type in the wrong disk name, and you can try to use the following command to check your disk name first.
Write down your disk name and type in the following command to check whether your disk can be ejected properly. Don't forget to replace 'disk0s2' with your disk name.
diskutil umount force /dev/disk0s2