FYI, it was a permissions issue - which I couldn't resolve. So, I came up with this WSH solution from here:
var objFSO = new ActiveXObject("Scripting.FileSystemObject");
for(var e = new Enumerator(objFSO.Drives); !e.atEnd(); e.moveNext()) {
var objDrive = e.item();
WScript.Echo("Available space: " + objDrive.AvailableSpace);
WScript.Echo("Drive letter: " + objDrive.DriveLetter);
WScript.Echo("Drive type: " + objDrive.DriveType);
WScript.Echo("File system: " + objDrive.FileSystem);
WScript.Echo("Free space: " + objDrive.FreeSpace);
WScript.Echo("Is ready: " + objDrive.IsReady);
WScript.Echo("Path: " + objDrive.Path);
WScript.Echo("Root folder: " + objDrive.RootFolder);
WScript.Echo("Serial number: " + objDrive.SerialNumber);
WScript.Echo("Share name: " + objDrive.ShareName);
WScript.Echo("Total size: " + objDrive.TotalSize);
WScript.Echo("Volume name: " + objDrive.VolumeName);
}