Let's say I have external drive, today no matter if it is mounted or not (but it is connected), I can see the icon on the desktop. Always the same. So I cannot tell by looking at its icon what the state is. I have to click RMB and check the available action -- if there is "mount" or "eject" present.

Please provide two kind of icons -- for mounted and not mounted drive. As the core it could be the same icon, but for example unmounted would be splitted with little gap in the middle, or for the mounted drive with little chain in the bottom. But those are only suggestions, the main point is, make them differ (do not rely only on colors, though).