Android's factory reset function isn't as effective as we'd all like it to be, according to a team of Cambridge University researchers. The group estimates that as many as 500 to 630 million Android devices might not be capable of completely wiping the data saved in their internal disks and SD cards. They came to that conclusion after testing 21 devices running Android 2.3 to 4.3 from five different manufacturers that already went through factory reset. During their tests, they were able to recover at least part of the data stored in each sample device -- even if it was protected with full-disk encryption.
The data they recovered includes contacts, images and videos, texts, emails and log-ins for third-party apps like Facebook and WhatsApp. They were also able to retrieve the master token needed to access all Google user data in 80 percent of the phones. There are many possible reasons for reset failure: according to the researchers, manufacturers sometimes don't load a phone's software with the drivers needed to completely wipe its internal disk or SD card add-on. Also, flash drives are notoriously difficult to erase.
It's unclear at the moment if Google or any of the manufacturers whose phones were tested are doing anything about this issue. But if you really want to protect your info before tossing, selling or giving away an old phone, use the most complicated password you can whip up... or, you know, go to town on it with a hammer.