Some manufactures document that they offer full data destruction on failed and returned hdd's to DOD standard.
Think i read it on HP's website, or Dells, i cant remember.
They did an article in Sound on Sound about recovering data from HDD's, they sledge hammered the drive, but using some type of scanning microscope or something they recovered 60% of the data.
Took ages, many bits of expensive kit, but shows they can get it back, if they really want to.