You can also see it in the Finder in the left column under Devices.
Mac hides your hard drive icons. If your Mac isn't set to show external disks in Finder or desktop, the external hard drive you inserted will not show up in macOS What does Recovery HD do. If you have a problem with a particular USB port, you might want to invest in a hub to make the most of your remaining connectivity. This is because while iCloud syncs the data, the relevant Mac folder icons become missing or hidden. If the Mac folder icons are missing on the desktop, then consider using Recoverit Data Recovery to get them back.
If it's not there, chances are it has been corrupted , is faulty, or is not formatted correctly. This article assumes you have an external drive that connects to your Mac via the USB or Firewire port.