One does reach a point where data can be destroyed, but that is also at cost. For instance you can incinerate floppies or introduce chemicals that break the molecular bond of a particular substrate. A bunch of random molecules in dried ooze won't provide much evidence. But one doesn't even have to go that far. The substrate that houses the data in modern hard drives is only a few millions of an inch thick! Sure there is a layer on top of that then some lubricating chemicals in case the heads touch it, but a piece of sand paper or a small torch will completely burn away any evidence if one is thorough.

Of course an electron microscope could find traces of data. But by then it would be out of order and just random? Hmm I guess that could be a case for NOT defragging? Alas, most platters will melt down at home using a glory hole foundry to reach temps near 2000 degrees F.

Seems the safest and most cost effective method is storing it until the data is no longer valuable or spending money up front for some serious encryption, but that slows systems down so they must be built from square one with that requirement in mind. But then it would be a shame to have it broken a year later.