If you are truly paranoid you should use an electric grinder on all the platters in each of the drives. Even the 35X overwrite can be read if someone wants to badly enough. Someone like NSA or FBI can do it but even they have to really want the data because it is so expensive and time consuming to recover information that is buried that deeply.
Yup, until it goes through the wood-chipper, it's still available, for a price. All you can hope to do is to make it more expensive than you're worth. I think if the FBI was going through your garbage can, the wood chipper would only be a good starting point.
When you really want to know what it takes to destroy something beyond state-actor-grade recovery, look to the military for ideas.
On a nuclear sub they still have paperwork to dispose of. They run it through a three tier shredder, which shreds it, then cross-cuts it.
They're not even close to done yet. Then it PULVERIZES it. i.e. turns it basically into powder. THEN they mix the dust with seawater to form a slurry, and pump THAT out of the sub.
Oh, and a truly interesting thing to muse about if you're bored.... what will future criminals, police, etc, 100 yrs from now think about our garbage dumps? Imagine recycling/reclamation tech that's going to exist at that point, they'll be considering former dumps and landfill sites to be the new oil fields in terms of recoverable materials. (imagine even all the gold you could recover from an entire landfill, if you had the tech? gold? meh. imagine the COPPER!) Now think what they would do if they run into a hdd that you've 3-passed. That may take them five minutes to read. Who knows what the future holds. A lot of people are going to seriously regret and sweat about things they threw away decades ago. We'll see a bit of a row over the ethics of going through old dumps, mark my words.
