For all platforms - VeraCrypt:
   [1]https://veracrypt.codeplex.com/
   Open source. Based on TrueCrypt with many security patches and
   improvements.
   VeraCrypt can create crypto-containers and encrypt the whole OS/Hard
   disc.
   Tested it on Lynux and Windows. Works great.
   Highly recommended.

References

   1. https://veracrypt.codeplex.com/