Diaries and private photos, personal emails, bank details and credit card numbers, passwords… Often we are not even aware of what personal and secret information our hard drives have stored. Just take for example the cookie that saves us from having to log into the online shop again, or all the passwords that the browser’s password manager has conveniently saved for us.

Thus, there are always sensational reports about computers or hard drives with highly confidential content being sold second-hand without having been sufficiently wiped beforehand. Another, somewhat more amusing example comes from the buyer of a used notebook which turned out to be defective. Since the seller was not willing to refund the money, the defrauded buyer published out of revenge all sorts of private and delicate details he found on the notebook’s hard drive.

Nevertheless, it can happen that you hand hard drives over to strangers because you want to sell, return, or dispose of them. How do you then securely and reliably wipe all confidential data?

A few important words beforehand!

This article refers to Linux systems and is mainly aimed at private individuals. Not because their data is less worthy of protection, but because the law requires professional and documented data erasure for commercially used hard drives containing personal data.

In this article, I also describe how data is securely and reliably wiped. With just one typo, data that was not supposed to be deleted can be destroyed in seconds. Therefore, you should pay close attention to whether the hard drive device is really the desired one, and rather look at the command one more time before pressing the Enter key. Important data that should not be deleted should always be backed up on a current backup.

In the following text, the hard drive to be wiped is addressed as /dev/sdX as an example. You should check in advance with hdparm -I /dev/sdX whether it is actually the hard drive model to be wiped.

Prevention is better than cure

This old wisdom also applies to sensitive data. It is better not to write it to the hard drive in plain text in the first place, than to have to remove it with effort later. If you store sensitive data encrypted on the hard drive, it is worthless without the key. This not only brings more security when selling a used hard drive, but also protects very effectively against nasty surprises after loss or theft.

Modern computers are fast enough to encrypt the entire system via LUKS without the performance noticeably suffering. Notebooks in particular should therefore always be set up fully encrypted with a secure password, even if that might be annoying when booting up the system.

But encryption is only one aspect. Even encrypted systems should ideally be wiped completely before you hand them over.

How not to do it…

What does not help is simply formatting the hard drive. This only recreates the management structures. “Undelete” programmes or data recovery services can recover a large part of the stored data from a previously formatted hard drive.

A low-level format is also not reliable. Some hard drive models ignore this command completely or do not function properly afterwards.

Commands like srm offer to securely delete individual files. One should also be sceptical of such tools, because there is no guarantee that the file system or the hard drive will not put a spanner in the works.

How do you do it right? For this, you have to distinguish whether it is a mechanical or electronic hard drive. With very old mechanical hard drives, it also gets a little more complex.

Wiping mechanical hard drives

With a classic hard drive, the data is written magnetically onto a rotating disk. Here it is sufficient to overwrite the entire hard drive once with zeroes to wipe all data. If partitions of the hard drive are still mounted, they are first unmounted. After that, the dd command handles the wiping process:

dd if=/dev/zero of=/dev/sdX bs=65536 status=progress

Depending on the age and size of the hard drive, this process takes several hours.

The strings command is suitable for checking whether a hard drive is empty.

strings /dev/sdX

The command outputs all readable character strings found on the hard drive. With a wiped hard drive, nothing of the sort should of course be found.

Wiping SSDs

Modern SSDs work purely electronically and store data in memory cells. Here too, the entire drive can be overwritten with zeroes as described above to wipe it reliably.

Unlike mechanical hard drives, however, the memory cells wear out with every write access. With many modern SSDs, manufacturers therefore resort to a trick to enable a gentle complete wipe. Here, the data is encrypted on the hardware side before it is written to the memory cells. For a complete wipe, it is sufficient to generate a new key. The data is then still present in the memory cells, but can no longer be decrypted even by the manufacturer or data recovery services.

This “Secure Erase” process can be carried out comfortably in the BIOS settings on many systems. It can also be carried out with the hdparm command, however a few things must be noted for this. On kernel.org there is a detailed article that exactly explains the necessary steps.

Wiping old hard drives

With very old hard drives, residual magnetisation can remain after a single overwrite with zeroes, which theoretically can be read out again under laboratory conditions. Here, several wiping passes are necessary, whereby random numbers as well as zeroes should be written to the hard drive, so that as many stored bits as possible change their magnetic state at least once. The US Department of Defence recommends overwriting the hard drive twice with zeroes and once with random numbers. It is even better to overwrite the hard drive seven times, of which random numbers are used at least three times.

The shred command simplifies this task, for example with the following line:

shred -n6 -z /dev/sdX

It overwrites the hard drive six times with random numbers (-n6) and then once more with zeroes (-z). Depending on the size of the hard drive, this can certainly take several days, but at least you can let the command run unobserved until it is finished.

With reasonably modern hard drives with a capacity of more than 100 GB, the data density is already so high that this possibility of restoration belongs in the realm of myths. Even the BSI (Germany’s Federal Office for Information Security) now considers a single overwrite with zeroes to be sufficiently secure.

What to do with defective hard drives?

Defective hard drives are a problem.

If individual sectors are defective, all modern hard drives automatically replace them with spare sectors. The defective sector is then no longer accessible and therefore cannot be wiped either. How many defective sectors the hard drive has replaced can be determined through the S.M.A.R.T. status. If at least one sector is defective, the hard drive should also be physically destroyed after wiping, just to be completely safe.

Completely defective hard drives (for example after a head crash or motor failure) cannot be wiped at all. Here, data recovery services are almost always able to read out large amounts of data. With defective hard drives, you will therefore not be able to avoid destruction in order to irretrievably wipe confidential data.

Destroying hard drives

Commercially used hard drives are best handed over to a certified service provider for destruction, in order to obtain a legally secure receipt that the data was professionally destroyed.

In a private setting, you can also destroy mechanical hard drives yourself by opening them and doing as much damage as possible with tools. It is usually sufficient to deform the magnetic platters. However, caution is advised, because some magnetic platters consist of thin glass and splinter very easily.

ATTENTION: There is a risk of injury from sharp edges and flying splinters! Always wear safety gloves and safety goggles!

With SSD drives, the memory chips must be destroyed, for example by drilling a hole in the middle of the chips with a sufficiently large drill bit.

Like all electronic items, hard drives do not belong in household waste, but must be disposed of via municipal collection points.