Do a guttman wipe on the drive, then hack it open with a saw, scrub throughly with sandpaper, and then drill a bunch of holes through the platters.
That will completely fuck any chance of salvaging private data alright.
I've dumped the platters in conc. sulfuric acid before, when I had to get rid of some HDs that the filth would have just loved to get their filthy trotters on.