We study maximal families W of space curves on a smooth cubic surface of any diameter with the restriction M1 = 0 and M3 0, where M is the Hartshorne Rao module. Under weak extra conditions, we prove that the closure of W in the Hilbert scheme H(d,g) of curves of degree d and arithmetic genus g is a non-reduced component provided g 3d - 18, thus proving a certain conjecture in this direction in most cases