OPC is correct, XP_FIXEDDRIVES cannot enumerate mounted volumes. A straight WMI call will also work using
wmic volume get capacity, "free space", name
You can also supply an output file and remote node names too
wmic /output:"c:\temp\mountsizes.txt" /node:sqlnode1,sqlnode2,sqlnode3,sqlnode4
volume get capacity, "free space", name
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"Ya can't make an omelette without breaking just a few eggs" 😉