A hard disk goes down in flames through the atmosphere and specialists were still able to recover 90 percent of the data: Data recovered from Seagate drive in Columbia shuttle disaster. At the end, data seems to be more resistant then most of us believe, when you put a vast amount of effort behind the recovery. This leads me to annother thoughtgame: Let´s assume, you shredder a disk to pieces as tiny as 1 square millimeter. This would just look like dust to you. A modern harddisk stores 200 gigabit per square inch. One square inch are 645.16 square milimeters. Thus a harddisk would store 310 megabits on a square milimeter. Let´s assume 10 bits per byte (for error correction and similar things) and you have 31 megabytes worth of data on one of these pieces of dust. It´s just a question of effort to recover the data, when you can yield 10 million euros out of the data (trade secrets, credit card data) it would give you a nice profit when you spend for example 9 million to recover the data. Yet another reason for cryptography everywhere or you may end up with degaussing, shreddering and remelt your old harddisks just to be safe.