Nothing lasts forever – especially not in the realm of hardware. If your Smartphone, Tablet or hard drive has long passed its prime and you have already made plans to sell it, you should make sure to thoroughly delete all data first. Here's how to do that for each individual device.

A study from 2012 found that a whopping 65% of all second hand hard drives are sold without scrubbing the data on it. While this might seem like a minor mishap at first, it actually poses a serious security risk for the seller if the device falls into the wrong hands. After all, any decent recovery tool can easily retrieve sensitive files containing social security numbers, passwords or bank account information within minutes. To prevent that from happening, its a good idea to consult specialized removal programs that can securely wipe possibly compromising information from your hard drive, laptop, smartphone, tablet, USB stick or SSD. Here's how to use them and what to keep in mind for each device. See also: How to clean up your PC for maximum performance

1. How safe are the standard deleting options?

Depending on the operating system of your soon-to-be-sold device, you might not need to resort to third party tools to do the job. That's why, in this section, we'll give an overview of the all the deletion options offered to users by default and explain how viable they are. If you want to jump straight into the action and see how to erase your files with dedicated tools however, head on to page 2 of this guide.

Deleting files while in Windows: It is common knowledge that Windows makes use of a two step deletion system: Redundant files first make their way into the recycle bin, typically located on the desktop, and can subsequently be irreversibly thrown off the system from there. What many users are not aware of however, is that this method actually doesn't delete the file from your hard drive at all – it just deletes all references to it.

This is due to the so-called “Master File Table” (MFT) of Windows, which is a hidden index that links files to their dedicated position on the hard drive and could thus be described as the table of contents for the whole partition. If the recycle bin is emptied, Windows turns to the MFT and simply removes the corresponding entry for the deleted files, thus losing access to the data without actually bothering to delete it.

While this form of deletion is quick and user-friendly on the one hand, it is easily reversible on the other one: Freely available recovery tools like Recuva can skim through the long stream of unreferenced data and re-establish “lost” files in a matter of minutes. Therefore, don't be lured into a false sense of security by emptying your recycle bin and always make sure to additionally clean up your hard drive with a more thorough approach.

Formatting with Windows: While relying on the Windows UI to ensure the safe removal of your data is a disaster waiting to happen, the formatting function is quite a different matter. Depending on which option you choose and what operating system you work with, it can either eradicate any trace of your data forever, or leave it lying on the hard drive in the same manner as the recycle bin.

By initializing the Windows formatting function, you will first get to choose from two different methods: Quick format and full format. The former is quite useless for our case, as it will merely delete the entries from the MFT file while leaving the data structure of the files themselves intact.

The latter option is not quite as easy to dismiss: If you are still using Windows XP, the full format option basically functions like a quick format in disguise, with the addition of a quick scan for bad sectors on your hard drive (not recommended). In Windows Vista and Windows 7 however, choosing the full format will indeed overwrite your whole hard drive with zeroes, thus making a recovery impossible. Using this option is therefore a viable alternative to third party tools.

Also take a look at: How to install a Windows 8 upgrade on an empty hard drive

Deleting files from the iPhone and the iPad: Apple uses the file system HFC+ on both devices. In a nutshell, this means that files on Apple devices are handled quite similarly to files in Windows: Upon deletion, they are moved to a hidden folder called “Trash” and once this folder is emptied, all contained files are marked as deleted and discarded. Naturally, they are just as easily recovered from the abyss as they would be under Windows, so that this function should be used with caution.

Deleting files from Android: Up to version 2.2, Android makes us of the unconventional YAFFS system („Yet Another Flash File System“), which records all modifications to files in the form of a log (with the newest changes being added at the very bottom). This includes operations like renaming, moving and even deleting, in which case Android will leave a corresponding note at the end of said log without overwriting the actual data.

Luckily, all remaining file leftovers will eventually be wiped of your Smartphone's storage by Android's “Garbage Collector”. According to a study by the ETH Zürich, deleted files survive for an average 44 hours after being deleted (as long as the smartphone is in use), thus almost nullifying the risk of data recovery.

Every since version 2.3, Android uses the more conventional file system Ext4. This functions quite similarly to Windows and handles the deletion of files in almost the same manner.

Doing a factory reset: A great number of Notebooks, Smartphones and Tablets hide a factory reset function within their operating system. Sounds great – but what this function exactly entails differs from device to device. In some cases, your hard drive will be formatted and an out-of-the-box version of your OS reinstalled. Others might just reinstall a new version of your OS immediately without bothering to delete the old files. To be safe, either look up the exact scope of the factory reset of your respective device, or rely on specialized tools as described in step 2.

Read on page two of this guide how to delete files with specialized tools.

2. How to delete files for good

Whether it be on Android, iOS or Windows - there is only one software-based method to banish files into non-existence: By overwriting their allocated space (or cells) on your memory medium with a different set of data. And since our guide doesn't advocate the use of jackhammers or bonfires to dispose of sensitive data, every method listed in the following paragraphs is ultimately based on just that approach. Naturally, this isn't to say that there's nothing else to worry about when overwriting your data - here how to do it properly.

A) Deleting individual files in Windows

To delete single or multiple files from Windows, you will need to get a hold of a dedicated removal tool, such as Secure Eraser.

Note: For better or worse, Windows Vista and 7 use a file backup system in the form of shadow copies. To ensure the complete removal of files, you will have to delete all of them before continuing on. To do so, head over “System > System Protection > Configure” and click on the button “Delete” (also deletes all Windows restore points).

Once that is done, open up Secure Eraser and select the option “File & Folder deletion” from the main menu. Select the files you wish to remove and click on “Start deletion”. On top of that, it's also advisable to clean up the free space on your drives to ensure that no previous shadow copies have survived (and to clean up all other hidden remains for that matter). You can do so by returning to the main menu of Secure Eraser and selecting “Free space deletion”.

Note: Unfortunately, all this is easier said than done with SSDs, USB sticks and SD cards. These are built on flash memory as opposed to the more conventional magnetic memory, which is much faster, but in turn also not quite as long-living as its predecessor. To counter that flaw and prolong their life span, flash memory distributes data fairly evenly across its whole capacity to level out cell usage. Thus, the attempt to overwrite files directly will more often than not just result in the creation of a new file in some other section of the drive, while the old file remains untouched – even with tools like Secure Eraser. As a consequence, the only safe method to wipe files from SSDs and Co. is to overwrite all the unused space thoroughly every time you want to cleanly remove a file. Obviously, this can't in good conscience be recommended due significant impact on the life span of flash-based devices.

This might also be of interest: Speed up your PC with an SSD - and the right tweaks

B) Wiping your hard drive clean

Hard drives without a system partition:

If the hard drive in question doesn't contain your system partition (if it isn't your primary hard drive and thus not home to Windows), you can scrub it quite easily without ever having to leave the Windows interface.

To get started, open up Secure Eraser and click on “Drive/partition deletion”. Select a partition that is housed on the respectable hard drive and click on “Start deletion”. This will cause a new menu to open, which asks you to define the security level for the deletion. It's perfectly fine to choose “Low – Random” for private purposes, but this is ultimately up to your own preference. After the operation has finished, select the other partitions on the hard drive and simply repeat the process until it is wiped clean.

Hard drives with a system partition:

If you wish to wipe your primary hard drive however, you will have to exit Windows first, as the operating system can't be deleted while it is still running. Obviously, make sure to evacuate all important files and folders to some other system, as you won't be able to resume working with this version of Windows afterwards.

If you want to go ahead, download the ISO image of Darik's Boot and Nuke and burn it onto a CD/DVD. Insert or connect the boot medium to your PC and reboot it. Make sure that your boot order prioritizes the respective device in the BIOS before proceeding.

Once you get to the main menu of Darik's Boot and Nuke, hit enter and wait a couple of seconds until you see a list of your hard drives. Select the one you want to scrub and mark it by hitting Space (this should add the label “wipe” in front of its name). Finally, to start the deletion process, press F10 and wait for the process to finish.

C) Resetting your Laptop

If you are planning to sell your whole Laptop with Windows intact, you often don't need to bother with the reinstallation manually: Most modern models offer a recovery option by default, which can completely reset your Laptop to factory settings with the push of a button. Look for a corresponding note in the corner of your screen when booting it up (often assigned to F10 or F11). By pressing it, you will enter a setup assistant that will guide you through the process.

Tip: It's a good idea never to trust the factory reset option to do the job of a proper removal program. To make sure everything is wiped clean, use the “Free space deletion” option of Secure Eraser as an additional measure after your Laptop has been reset.

D) Scrubbing smartphones and tablets

Whether it be Android or iOS, the first step to prepare your device for sale is always a factory reset. In the former case, you can do so be heading to “Settings” → “Privacy” → “Factory data reset”. Make sure to check the option “Format SD card /Erase SD card to delete all of your files and settings and click on “Reset phone/device”. If you are using an Apple device, navigate along the path “Settings” > “General” > “Reset” > “Erase all Content and Settings” and enter your iOS password, if you have set one. Confirm the action by tapping on “Erase Phone/Tablet”.

Android: As downloading any app without a Google account is impossible, you won't get around entering some personal information to scrub your phone/tablet. It is therefore advisable to set up a dummy account for the time being, as Android logs all mail addresses as a matter of principle and won't dispose of them easily. Look for the free app Shredroid on Google Play and install it on your device. Start it up and select “External Storage” as well as “Internal Storage” and tap on “Secure Delete now”. After the process has finished, it's a good idea to initialize a factory reset again in order to get rid of the app and your digital fingerprints.

Note: Don't use version 1.1.6 of Shredroid in combination with a device from Motorola, as this might cause unexpected errors and crashes.

iOS: As with Android, it's impossible to avoid entering some personal details to get a hold of removal tools. Log into the App store with your Apple-ID and look for the app iShredder (priced at £2,49). Install it and look for the little gear symbol on the bottom right. Click on it and you will get to choose between different algorithms for the deletion. Select “1 cycle: Random numbers” and hit the play symbol at the bottom right to start the process. As iOS won't save your Apple ID password along with your account, your mail address will be the only thing that remains retrievable after the process has finished. And though this doesn't pose a security risk, you might want to reset the phone again to make it squeaky clean.

This article is based on a segment by our sister publiscation