If you're going to go that route, you want an encrypted hard drive whose keys can be destroyed at a moment's notice. I believe that's off-the-shelf tech now, but I'm not sure where to point you at it.

It would also have to have no recovery mechanism, otherwise that could be (ab)used to recover data after nuke.

What's the point of this? You'd only ever use it if you were about to have it seized, and doing so would be destruction of evidence.

watch this years defcon presentation by Zoz, destroying hdd reliably is a difficult task