2 Mac HD's on the desktop?

I just did a backup of my laptop and when it finished I moved the laptop slightly and a second HD icon appeared on the desktop. What does this mean? Can I safely shut down without losing everything/ anything?

Hope someone can advise me what to do about the second icon?

Thanks.

After looking on-line again and phrasing my question differently, I found an answer and restarted and the second icon disappeared. Whew!