Hi there, you can go to check if Finder->Settings->Sidebar that "External Disks" is checked. If you did, try to delete Finder preferences to make the external hard drive show up in Finder sidebar, just press Command-Shift-G in Finder window and paste the following path to enter it:
~/Library/Preferences/com.apple.finder.plist
Then, drag the targeted file to Mac Trash. After this, go to see if any external hard drive show up in Finder sidebar.
For more info, please visit: Fix External Hard Drive Not Showing up or Recognized on Mac
