Re: How many overwrites for secure erase?

Unruh wrote:

1. read the data as a analogue signal


Move the platers into a special reader which gives you the signal directly, or use a laser.

2. interpret the signal as digital signal
3. calculate the ideal analogue signal that represents these digital data
4. calculate the difference between the two, the result is the overlap of the previous signal(s) and noise
Yes, noise dominates.
(5. repeat)

What good does that do. The noise is there on the platter And because of
the very complex data encoding that "signal " has only an indirect
relation to the actual data.

You can apply the exact decoding and error correction to extract both the most likely data and their confidence. Which, even in presence of strong noise, works very well with Turbo-like Codes.

But the Turbo-Like Codes made it possible to get within 10% of the Shannon limit while still remaining a very low bit error rate, that's why today there's not much redundancy left to exploit.

AFAIK many companies actually offer this procedure if more trivial measures fail, but for a horrible price.

Perhaps you could tell us some, and tell us how you know they offer this

As I already said: If you ever had something that couldn't be recovered, they offer you such drastic measures for about 10 to 100 times the price. Almost any customer can tel you this, and even PC World and Heise have already experienced this (because they sent in a hard drive overwritten with zeros; sadly they refused to try the better recovery mechanisms).

This is nothing special though, even Peter Gutmann mentions much more potent ways like magnetic force scanning tunneling microscopy.