Does anyone know how or if it is possible to show "used space" instead of "free space" attached to the drive icons in "My Computer"/"This PC"?

Hi,

I dont think so, you can't change the information being presented there.

It was designed like that, and the computer user as no control over the information presented at that specific location. 

You can however see that information on the same window activating at least one of the following two options:

This will activate the Percent Full that shows a nice graphical representation as a bar, that will be shown both in the Details and Tiles view.

In this computer view, press alt + v to open the view menu

Click on the Choose details... item

You will see that Percent Full is one of the available items, Tick it.

Note: this won't be shown for removable media drives.

Regards,

D. Wu