You would think that when you do a “factory reset” and agree to erase everything on your Android device, all your data would disappear. But you would be wrong!
As many researchers have discovered, data can be recovered with the right tools, even after a factory reset. That's because when you delete something, the file itself isn't actually overwritten — the system just throws away all the information about the file. Recoverable data can be a real problem if you ever plan to sell or give away your Android.
However, there are a few ways to ensure that your data is truly deleted once and for all.
Encrypt your phone
Encrypting your device is the strongest way to prevent data from being recovered. By encrypting your device, you are essentially encrypting all of your data and locking it with a special key. Once encrypted, the data cannot be decrypted without entering a password.
Most devices running Android 6.0 Marshmallow are required by Google to have mandatory encryption for maximum security.
If your device is running Android 5.0 Lollipop or older (and supports encryption), it is recommended that you enable encryption (Settings > Security > Encrypt phone) to scramble its data before performing a factory reset. This essentially erases encrypted data that is virtually unrecoverable.
Replace with disposable data
So after encrypting your data, factory resetting and wiping everything, let's go even further to make sure the data has been rendered useless and unrecoverable.
Go ahead and set up your device again, but this time don't sign up for any other Google accounts or anything. Once your device is set up as new, shoot a video for as long as you can, at the highest resolution. Repeat as many times as necessary to fill all the space. Then do another factory reset.
Repeat
You can follow the above tips and run them several times until you feel confident that the data is encoded enough and will be replaced with new useless data that even if someone were able to recover it, they would not get anything useful.
