The best way is to use a simple WMI query as follows

wmic volume get capacity, "free space", name

If xp_cmdshell is disabled you can turn it on first then disable it again afterwards.

Or you could run it from a central trusted server where xp_cmdshell is on and use the Node paarameter to read remote servers like so (also has an output file)

wmic /output:"c:\temp\mountsizes.txt" /node:sqlnode1,sqlnode2,sqlnode3,sqlnode4

volume get capacity, "free space", name, systemname