In Mac OS when you try to remove an USB drive it displays “The disk was not ejected properly. If possible always eject a disk before unplugging it or tuning it off“.
This error message is displayed when you have not ejected USB drive properly. In order to properly remove an USB drive on a Mac OS, Launch the Finder Tool and right click on the USB disk that needs to be removed. And select Eject <disk name> from the displayed list.
Now you can safely remove your USB drive from Mac.