Recovering overwritten data - can software alone do it?

[kythe]

well, over written data can be recovered with special hardware. Most easily if you know what it was over written with. For example say a 0 on a hard drive all the particles are aligned - and a 1 is |. If you over write all data on a drive with a single pass of just 0s, most of the particles will be -, however just a few will be | still where the 1s were. Get sensetive enough equipment and you can detect that.

Theoretically, it's possible. But again, when researchers have actually tried to do it, the signal is buried in the noise--more sensitivity doesn't help.

Usually, what's done is to attach the raw analog signal output from the read heads to a spectrum analyzer or oscilloscope (or similar test equipment). If you do that, you can see the old data if there's only been one overwrite pass. Of course, you have to read the same area 100 times to get the noise low enough to see the signal, the old data has to be a repeating pattern, and again, you have to know the overwrite pattern (always use a pseudo-random overwrite pattern!). And after more than one overwrite pass, the signal disappears, no matter what crutches you may have for finding it.

Here's a great example:

http://www.tomcoughlin.com/Techpaper...,%20042502.pdf

This is one of those things that's often been rumored, but never demonstrated in practice. Everyone who has tried to find someone who can actually recover data in this way has come up disappointed, and most conclude it's an urban legend.

The government, out of an abundance of caution, recommends overwriting with multiple passes. If that makes you more comfortable, do that.