"The FBI has their ways of getting whatever they want"
There are precautions that can be taken. Like psychos who cross shed documents 1/16", incenerate them, mix them in a large vat of water, then pump them into the ocean off the contenential shelf. And just think: somewhere, someone asked an engineer "what's the best way to get rid of top secret documents", and the guy came up with *THAT* idea.
Taking your friendly hard disk into the nearest MRI room (while it's not on, mind you), and encasing it inside 4 duffel bags then using cargo tie-down straps to secure it to the large wooden table then waiting for the next patient to mosey in, comes to mind.
Other than that, erasing, filling, erasing, filling, erasing, filling, erasing, filling, erasing, filling (is that 5 times yet?) would probably get 99%.