Sure way to ensure data is irretrievable is what I saw in actual use at a major bank: shred the hardware.
Seen those big shredding machines for turning large tree limbs into wood chips (also used to shred plastic and metal for recycling)?
That's the machines you want. Turn them on, set at a nicely small granularity (say half a centimeter), enter the disks (and other components, like CPUs, motherboards, network cards) into the hopper, and stand back.
The resulting fragments are guaranteed to be impossible to turn back into a working device (and if you're more paranoid still, add an oven and melt them down).