Hi there, I've glanced over all the dialogues you have with others and all the attempts you have tried. From my experience, I can only give a few solutions for you to solve your problem. While among all my solutions, a computer restart is the simplest one.
Try a self-powered hub. It is possible that there is something thing wrong with the USB power. In such a case, you can connect an external drive through a separate power supply to your computer to check if the trouble is fixed.
Or, you might ask the machine to hide the external hard drives so you need to show them manually by launching Finder on your computer > heading to the Sidebar > placing the cursor just to the right of the word "Devices" in the sidebar > click the Show button.