Protect Your Data When You Sell Your Computer

Someone contacted me last Friday with some questions regarding selling a computer, and it made me realize that a tip on protecting your data before you sell your device would be a great idea.  Identity theft is a major problem, and it is important that you take the steps to completely remove your information before you sell a device, especially if you are selling to someone you do not know.

For a laptop or desktop computer, I recommend a two-step approach.  First, remove the data files from the device by deleting them or by doing a factory reset.  To delete the files, simply move them to the recycle bin or trash can and then empty it.  For a more secure approach, you can do a factory reset to restore your device to its "out-of-the-box" state.  Most computers will have built-in software to perform this operation, but you will have to follow the instructions in the user manual since each computer has a different process for starting a factory reset.  (Some manufacturers may use other terms, such as "System Restore" for this process.)  This process will delete everything on the drive and then reinstall Windows.

However, even a factory reset does not mean that your files are completely gone.  When you delete a file, the computer does not actually remove the data; it just marks that area of the drive as usable.  The files are still there, and they can still be recovered and pieced together.  The reinstall of the operating system will write new data over some of your files, but there will still be areas with leftover data.

To remove this data, I recommend using the free program CCleaner.  Along with many other tools, it contains a drive wiper that can wipe all the free space on the drive.  After installing, click on Tools and then Drive Wiper.  Choose Free Space Only (or if it is installed as a second drive on another computer, you can choose Entire Drive), the number of times you want data written into each area of the drive, and the drive or drives you want to wipe.  Click the wipe button, and let CCleaner go to work.  Depending on the amount of free space on the drive and the number of passes selected, this may take a substantial amount of time, so you may want to let it run overnight.

If you are selling a phone or tablet, you will also want to ensure your data is protected.  Fortunately, both iOS and Android have built-in settings to do this.  iOS devices will do this by either encrypting all data and deleting the encryption key or by overwriting all data on the device.  Simply navigate to Settings, then to General and Reset, and select the Erase All Content and Settings option.

With Android devices, the process is slightly more complicated.  You will need to encrypt your data before performing the reset.  (Avast recently conducted a study by purchasing 20 phones that had been wiped to factory settings and found that it could recover data even after the factory reset.)  First, set up a lock screen PIN or password by going to Settings, Security, and Screen Lock.  Then, go to Settings, Security, and choose Encrypt Phone.  You will be prompted to enter your PIN and respond to a warning message.

Now, you can do the factory reset.  Go to Settings, Privacy, and choose the Factory Data Reset option.  The phone will now delete the encrypted data along with the encryption key.

Are Your Files Really Deleted?

You've selected your file and hit the delete key, and then you've gone to the recycle bin (or trash can) and emptied it. Your file has now entered the world of electronic oblivion, right?

Not so fast. Every single piece of your data is still there. Obviously, this is good news if you accidentally deleted a file that you now want back, but it not good if you are planning to sell or dispose of your computer and do not want your personal information in the wrong hands.

To understand this, your hard drive is divided into small sectors. When you save a file, it places it in one of these sectors. If it is too big for one sector, it will fill the first sector, and at the very end will be code telling it where to look for the next sequence of data. Very large files may fill tens or hundreds of sectors. The file system in your operating system (Windows, MacOS, Linux, etc.) contains a list of the files saved and where to look for the first sector of the file. When you open a file, the computer looks in the file system for the first sector, loads that sector, and then continues the process with each sector the file is using.

At the very beginning of each sector is a single bit that tells the computer whether or not it is allowed to write data into that sector. When you empty the recycle bin, it simply deletes the reference to the file in the file system and changes all these bits so that the computer knows it can write new data into that sector. However, the data is actually still there! Over time, much of it will eventually be written over by new files, but until the computer decided it needs that sector for new data, your old data will still be stored there.

However, you can make sure your files are actually gone. If you are planning to sell or dispose of your computer, I would recommend using the program CCleaner. The free version contains a Drive Wiper. Once it opens, click on tools on the left side, and then select drive wiper. Choose to wipe the free space (wiping the entire drive requires taking the drive out of your computer and putting it into another computer), and then choose the security level: the number of times that the program will write junk data into each empty sector. A single pass will be enough for most purposes, but if you have highly-sensitive data, you may want to choose more passes. Then, select the drive you want to clean, and click Wipe. The process will take a while, but the protection you will receive is definitely worth it.