Wiping a Hard Drive

The safe thing to do:
1. Be sure you have the recovery disks that came with the laptop or that you can make (by running a program from the hard disk).
2. Download DBAN (a .iso-file) as in image to the hard disk (NOT as a file). Use the 'burn image' mode of your favorite burning program or download IMGBURN to do it. Both programs are free.
3. Boot from that disk and say OK or yes to confirm the wiping of your drive,
4. Boot from the recovery disk and install that.
Now your laptop is brand new.

Kees

The safest method is not to sell the laptop with a HDD at all, since no matter how many times you try and erase a drive, there's always a chance someone might be able to get data off it.

That being said, after you use something like DBAN and let it run for a few days, the odds of anyone with resources less than a government or Bill Gates being able to get at the data are slim, but they do exist.

if the laptop manufacturer provided either disks or a partition on the hard drive that contain an image that restores the computer to factory condition all you have to do is use that image to restore to factory condition.

That will effectively wipe the contents of the drive and replace it with the image - that is why an image does not require partitioning or formatting the drive.

Doing this is every bit as effective as DBAN and makes any type of file recovery a clean room process that is only economically feasible for REALLY important things like State Secrets and any type of overwriting of the drive leaves that type of file recovery available through what is commonly known as magnetic remembrance. Examination of the disk with an electron microscope can still reveal the previous contents of the wiped area, because the obliterating bytes are not written in exactly the same tracks as the original data and there is still a little of the original data left around the edges. This is why government hard drives that are classified as anything more than confidential require physical destruction rather than overwriting.

For your purposes simply returning the computer to factory specs adequately does the trick unless this or some other government is really interested in what was on the drive.
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There is reasonable precaution and extreme precaution. If you've not buried a body somewhere and mapped the location on your hard drive, you should be able to go with what's reasonable. Another method I will use is just get the manufacturer's destructive testing utility. You'd need to know the hard drive's manufacturer. Dban is extremely slow but very thorough. A manufacturer's utility that writes zeros to all locations followed by a re-installation of the original image will probably be better than 99.99% effective and could save you hours of waiting time. In the end, the choice is yours but paranoia isn't what needs to be the driver.