Unlike CDs, removable drives are not ejected. Think "Safely Remove". If the context menu offers you only the possibility to mount, then you are not mounted and it is safe to remove the device. If the device is mounted you will be offered the "Safely Remove" option. That flushes any buffers in use, so that you don't lose data when you remove it. The same options can be seen in the Device Notifier, where there is a small plug icon if the device is unmounted, and an eject icon (up-arrow in a circle, similar to on video players) when it is mounted. Always allow a few seconds to ensure that nothing is still writing to your drives.
KDE UserBase Wiki 