DriveSpace Mk2
It is called Mk2 to align with the Widget of similar functionality I built under the XWidget engine a while ago.

The widget is self adjusting vertically as it detects drives being added or removed, or hidden via the configuration panel, keeping the drives in order from C through to Z.
The ability to "Hide" a drive may seem odd, but sometimes certain drives serve on a specialised function and are not needed for 'day to day' operation; for instance, my drive D:\ is a recovery drive for when the computer is to be re-imaged back to factory settings, not the kind of drive I would need to access or monitor. I also hide 'back-up' drives.
Thanks also to yincognito for direction to the solution to get the bar colour to transition based on usage.
Ideally, it would be better if the optical drive bar size and capacity values could reflect "real" values. But that is out of scope for Rainmeter currently. It does, amusingly, fly in the face of convention to show a full bar on a zero value…