Not seeing external drive on desktop

Go to Finder > Preferences and make sure that the boxes for Desktop and Sidebar are checked.

https://support.apple.com/guide/mac-help/finder-preferences-mchlp2803/10.13/mac/10.13

since you are out of ideas then try this:

power off and disconnect all devices and then

SMC reset      https://support.apple.com/en-us/HT201295

Zap pram   https://support.apple.com/en-us/HT204063

reconnect your devices.