@topic: I would strongly suggest using a hardware key that also utilizes a passphrase. To delete, remove the key and/or don't tell anyone the passphrase. If you need to destroy a platter drive take it apart and sand the platters (probably the easiest). If it's solid state heat the drive over 150C-250C for an extended period of time or mechanically destroy the chips.
>It contains no theoretical arguments against the possibility of data recovery. The superparmagnetic limit sets the upper bound for storage density. It is impossible to store information inside the grain of a metal because it acts as if the magnetic moment is the sum of all of the atoms in the grain. At this size, the polarity of the magnet can randomly flip directions depending on the temperature. For ~2005 drives that was about 1Tbit/in^2 with ~850Gbit/in^2 used. Newer drives continue to have higher numbers but unless the efficiency drops there is not enough room to shadow all the data (you will need to calculate or find these numbers for each drive you are interested in). At best you could hope to recover some portion of it with magnetic force microscopy, which you can/should assume will read back at the maximum density available on the medium. But, simpler: if you combine a random stream of data with what is on the drive, the result looks just like random data. You need only overwrite the drive once.
