Beyond that, though, isn't empty space encryption far more important as respects SSDs than it is as respects HDDs because of the fact that SSDs retain data even after they've been "erased"?
Who said magnetic HDs don't retain any data when they have been erased? It all depends on how badly someone wants the data. Even the 7x overwrite is NOT foolproof. There are labs that can go deeper than that. The only foolproof erasure is a grinder that reduces the platter to dust.
