Possibility of recovering files from a dd zero-filled hard disk

As you can read here, it is impossible to recover data after you "zero filled" it.

There can be a chance of 56% to recover a single bit correctly, but since you had to recover 8bit to get only one byte, it is very unlikely to recover any data.

Be very careful with this information. I work in the HDD industry and I CAN confirm that off-track reads can recover over-written data.

Some recovery methods use this trick to set the head +/-10% off-track, then read, move it off-track a little more, then read. At some point you will be able to recover what was laid down before the zero fill.

Use random when possible. Zero is okay for meta-data and MBR erasure. I recommend several random passes to obliterate the original data.

Also, zero does not mean cleared recorded bits on an HDD. Zero has a bit pattern just like any other number.

Yeah... But it depends how paranoid you are.

A professional could probably still read some of the data. Government/military standards for "completely wiping" entail several passes including writing random data over the entire drive several times, interspersed with 0-fills and 1-fills. This is because there is magnetic ghosting that sophisticated hardware can analyse and pull out. This is expensive kit that most people won't have access to and therefore just hiring somebody to do the extraction is also prohibitively expensive for most people.

But there's no reason dd alone can't do these multiple passes. You can tell it where to source the raw data it writes so alternating between /dev/random and zero- and one- passes would, I think, qualify it to do pretty substantial damage to data.