That normally occurs when something on the drive is still in use - although mine will occasionally do this even with evrything closed.
On these occasions I have pulled the drive with no consequence
Samsung Galaxy S8 review
1995-2015: How technology has changed the world in 20 years
This poetic animation about monsters is illustrated in a Van Gogh painterly style
Apple predictions 2017
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.