The only disk encryption tool that was widely available to the public in source code and was audited by an independent third-party security laboratory is TrueCrypt. Unfortunately, the authors of TrueCrypt stopped the development in May 2015 and proposed an outrageous in its insecurity move to built-in encryption on Windows. Knowing that we do not trust closed source systems, knowing that all large companies collaborate with governments to insert backdoors to their software, knowing that there is a large interest in corporation to subvert user privacy, I suggest you keep to the TrueCrypt. Hopefully someone will step up and continue the development but the good old versions still work and are available here at: https://holyhash.com/truecrypt/. The version audited by iSec Partners is 7.1a and that is the recommended one for the moment.