It's not secure. If you're looking for a secure erase of the internal storage, you need to encrypt your information, then perform a factory reset. When encrypting information, you need to provide a pin number. The only way to decrypt the information is with that pin number. When you factory reset, the encrypted information may still be retrievable from the internal storage after the storage has been wiped, but anyone attempting to read a file will get garbage unless they have the pin. As I pointed out in the previous thread where I mentioned this, unless you have stuff you seriously need to hide, there's no point in using this method.