Your question got my mind of thinking. I have tested the script on a Lab server to verify it would not blow up. Currently, we do not have any multiple instance servers.

So you are running 12 instances on one Windows Cluster. Is the problem the same drive is being reported twelve times?

Or you want to see only drives listed with its assigned instance?

It will take me some time to think about it. I do not know of any other scripts to collect space on drives. What release of SQL are you using? Maybe 2008 has a DMV.