If your standard is "keep them from wiping it so thoroughly that it cannot be recovered by experts that exist all over the country," then you can take an hour or more to get in, if you like
Your mistake is that you're assuming a method to be used that gives you a still-functional HD at the end of it. If police are at the door I don't care about saving some pissant $60 hardware device that might put me away for X years. My illegal porn/drug notes/fake/stolen ID's/stolen classified documents/conspiracy theories/etc... are going away.
If I'm set up, you have at most 30 seconds before not even the NSA can recover it.
Options:
1. Permanent magnet degausser - take loose HD, feed through, gone. Takes ~15 seconds once I start cranking.
2. HD is in a heavy ceramic urn(using an extension cable) with thermite piled over it and an ignition device wired to a button(and/or timer). Good luck recovering it while the thermite is still going...
3. HD is wrapped in coils hooked up to a switch on household ac power.
4. HD has a key secured truecrypt partition. Destroy ~2k of information, the rest of the HD is useless. Simply writing over the key would be enough. One option is to store the key in a RAM device where if power is cut it goes by-by. Hitting the button/yanking the cord powers down the system and renders all unaccessable. There are usb-key devices that will erase themselves if you push a button.