To read this paper to be presented at the Usenix HotSec Workshop, on 29 July in San Jose, CA, by Alexei Czeskis, David J. St. Hilaire, Karl Koscher, Steven D. Gribble, Tadayoshi Kohno (Dept. of Computer Science and Engineering, Univ. of Washington) and Bruce Schneier (BT), click on:

Encrypted HDDs not so Safe ?
Defeating Encrypted and Deniable File Systems:
TrueCrypt v5.1a and the Case of the Tattling OS and Applications

Look also to TrueCrypt, a free open-source disk encryption software.