Returns map {volume_id: mount_point}

def _mount_points(self):
        """ Returns map {volume_id: mount_point} """
        diskutil_ls = subprocess.Popen(
            ["diskutil", "list", "-plist"], stdout=subprocess.PIPE
        )
        disks = _plist_from_popen(diskutil_ls)

        return {
            disk["DeviceIdentifier"]: disk.get("MountPoint", None)
            for disk in disks["AllDisksAndPartitions"]
        }