Go via Inventory - Mass Storage - Logical drive - available storage and combine with the drive letter that you want to check. Note that free space is measured in MB!
Well, Frank . . . . thank you, even though I now feel very stupid.
I actually did look at "Mass Storage - Logical drive - available storage" before, but I completely misinterpreted it. I was thinking that meant "hard drive size", and not available space. If I had only looked a litte more closely at the scanned values, I would have realized it was exactly what I needed.