'Modifies server hard disk drives
:param server_id: Id of the server
:rtype: ``str``
:param hdd_id: Id of the hard disk
:rtype: ``str``
:param size: Size of the hard disk
:rtype: ``str``
:return:    Instance of class ``Node``
:rtype:     :class: `Node`'

def ex_modify_server_hdd(self, server_id, hdd_id=None, size=None):

    body = {}
    if (size is not None):
        body['size'] = size
        response = self.connection.request(action=('servers/%s/hardware/hdds/%s' % (server_id, hdd_id)), data=body, method='PUT')
        return self._to_node(response.object)
