Do you want to give your Android smartphone or tablet, but be afraid of your files being exposed? Below we will see how you will make sure that all your data has been deleted. 
Before you begin, keep a backup of all your data, because as you understand if your files are deleted, they left and there is no undo.
Then plug the charger into device because the deletion process may take some time.
From Settings> Security> Phone Encryption (or Tablet Encryption) to get started. On Android 4.4 or older devices go to Settings> Security> Screen Lock to start the process.
You should guide the process. Set a strong password. You do not have to encrypt the SD card because you can just remove it and format it. Encryption can take longer than an hour.
Immediately after Settings> Backup & Restore> Factory Data Restore, then tap Reset tablet.
Once this process has been completed, your data has been deleted.
From a technical point of view, however, it is still described as free space. But if someone tries to retrieve your data they are encrypted.
However, if you want to delete them completely, proceed to the next step.
The best way to get rid of the encrypted data is to replace it with new data.
There are two different ways you can do this.
You can simply upload large files to your device (such as video files that are ideal for size) until the disc. Then delete the files you added.
Μπορείτε επίσης να χρησιμοποιήσετε μια εφαρμογή όπως τη iShredder που μπορεί να "τεμαχίσει" ψηφιακά τα δεδομένα.
That's it! Your data has disappeared for good.
