Simple question: A friend of mine is going to sell her laptop online and she heard about people recovering data from the harddrive with harddrive-recovery tools out there on the interwebs. No matter how small the chance anyone is going to do that, I think she is right. So is there a program out there that just simply clears all the data on the harddrive and set it all to an arbitrary value? Or is a full (not quick) format + reinstall of OS enough?

A format would likely be enough, but http://www.dban.org/ is quick and easy.

Why would someone spend money, time and effort to recover files from her laptop. Does she work for the CIA or something.

I'd say dban too, but I'm fairly sure cCleaner can wipe everything not currently installed.

The Great Zero Challenge offers $500 for anyone that can recover data from a zeroed drive. Unfortunately, the only thing that can do that is probably an electron microscope, the operation of which would cost far more than $500.

Use DBAN if you're paranoid, but zeroing the drive should prevent anyone other than the NSA from getting data off it.

Actually, the use of an electron microscope was only proposed in one paper in 1996, and the theory was greatly criticized. It hasn't actually been done. I think it's safe to say that the NSA would be hard pressed to recover anything from a completely overwritten drive.

What would the data actually exist as then? If you've zeroed the drive, and over written it a few times, what could possibly remain that you could reconstruct to anything?

ATOMS!

I have had some fun times with 2nd hand computers and data left on it by previous owners.

One time I got a used computer from a Computer Retail store (purposely not named but you can probably guess) and they didnt bother to re-format/reinstall. When I first booted it up, it logged into the previous owners AIM, Facebook, ect. It also had a large collection of mp3's and movies.

Another time I ordered a computer off Ebay which turns out was previously used as a mall photo kiosk. Apparently the owner had made no effort to remove the photos, some of the photos were quite amusing.

So always format and overwrite!

SCIENCE!

That is really the best pic I could find.

Oh, okay, well whatever high end methods are used by data recovery companies, they probably cost a lot more than the $500 prize.

Well, beyond that, it simply can't be done, so they wouldn't be able to get it anyway.

I think a good analogy would be this,- Write on a piece of paper with a pencil, erase it, then cover the area with the pencil.
Repeat this several times.
E: Actually a whiteboard and pen would be more apt.

The NSA/etc government facilities have tech to recover data from drives that have been formatted once. Note that this is incredibly time consuming and highly advanced software, and still isn't perfect. If you format it twice, it becomes an order of magnitude even harder (to the point where they wouldn't bother even trying).

So in other words, a plain format is good for most, a 2 run format is good for everyone. Unless you're the government, in which case you do things "by the book" and erase it something like 7 times before bombing it with napalm and dropping it in the ocean.

It is possible to determine individual bits even after multiple rewrites, a given pattern of 0/1 writes leaves a sort of EM signature for that pattern, but it's ridiculously hard to get, to the point where it's NSA-level if they really want to.

It USED TO BE possible to do stuff like that.

It's not anymore.

I haven't searched too deeply but there seems to be a general lack of information on whether or not it's possible on modern hard drives. Whether or not anyone has ever done it.

Well the idea is that the head won't 100% line up over the old bit when it writes over it, so there will be little edges that could be read with a really powerful electron microscope, but the difficulty involved would make useful data recovery completely impractical with any hardware. The NSA does not have this capability.

Perhaps in decades past, but the bits are so damn close on modern drives that I doubt anything could be recovered.