When using macOS, you certainly know if you’ve opened a Pages file or are editing an email. But, below the surface are files that macOS uses that you don’t see–all the activity required for background processes, Time Machine backups, Spotlight indexing, and other system needs. When you want to eject a mounted volume, like a volume on an external SSD or hard drive, you can run afoul of macOS’s hidden needs.
You can try to eject a volume normally in these ways:
• Select it in the Finder and press Command-E.
• Select it in the Finder and choose File > Eject.
• Control-click/right-click the volume in the Finder and choose Eject “volume name.”
(If you don’t see the volume in the Finder, either choose Go > Computer or press Command-Shift-C, or choose Finder > Settings > General and check all the items under “Show these items on the desktop.”)
Read more at Macworld.com