If you look in the system tray you might see the icon a small grey PC with a green tick on it. If you don't see it, click the small arrow by the system tray and choose 'customise' to set 'Show icon and notifications' for the 'Safely Remove Hardware' icon.
Then simply click the icon in the system tray and select 'Eject Docking Station'.
Having said all that, I seem to often get a message that the disk is still in use - I have no idea what is using it and there is no clue of course ...