Even sold secure gold d-locks can be cut with angle grinders. The bigger/better ones can't be cut with even big bolt cutters though and that's part of the sold secure rating.Saw what was labelled as a Kryptonite lock on a cycle stand a few days ago; the shackle had been neatly cut through.
The Litelok and the big Hiplok locks are resistant to angle grinders due to other materials they include in the shackle. Resistant, not proof of course.