Although it was another lifetime ago, I used to work at Apple, so when I have issues with my Mac, it bruises my ego. Lucky for me, the internets are full of answers to questions. My hard drive disappeared from the finder. I knew it was ok, I could get to it through the Terminal, but how to get it back up in my finder window. Here’s the answer I needed today:
“Show missing hard disks in the Finder sidebar
The Finder sidebar is a convenient location to access frequently used items such as hard drives, various folders in your home directory, network shares, and any custom items you drag there. If you use the sidebar for accessing hard disks, then there may be times when certain hard disks do not show up here, while others do.
The likely reason for this occurring is if you have removed that specific hard drive from the sidebar. Even though the Finder has a sidebar setting you can check or uncheck to globally show or hide all external hard disks, there is a gray area where only some of these enabled items will be shown. This happens when you select and drag items off of your sidebar manually.”
https://www.macissues.com/2014/10/23/show-missing-hard-disks-in-the-finder-sidebar/