For that to work you have to trust the firmware. Overwriting with something random, saving what was fed and then cross-referencing that against the storage coul

For that to work you have to trust the firmware. Overwriting with something random, saving what was fed and then cross-referencing that against the storage could work better, but there's still some non-zero chance that something you're looking for is in a buffer, unreachable part of the disk or the like. Encrypting the hard drive and then removing the key has a better chance of rendering the data unusable.