This plugin will display a list of items representing your various mounted and unmounted disk devices. If you click on an unmounted devices it will mount it and vice versa. There is no warning in case a device can't be mounted, but there is one when unmounting fails. User needs proper doas(1) rights to mount/umount/eject devices.
WWW: http://goodies.xfce.org/projects/panel-plugins/xfce4-mount-plugin