I suggest a heavy earth magnet because the truly paranoid know that new drives store all your secrets into the low level DRM partition that we mortals can't reach :-)
On a slightly serious note, I wonder if this is one of the reasons PGP suggests nine passes to wipe a disk? Whatever type of cache is used for something like this can only hold so much data, and this could potentially push your, uhm, "evidence" out of the cache.