I am pretty sure that many FDE solutions only encrypt things that are currently indexed, and not the entire drive. That is why they generally have an option included to do a wipe prior to encryption. Here are some snippets supporting this though: As you can see, Bitlocker does not encrypt the free space on the drive. But they take care of the problem by overwriting it. However my point is that the encryption process and the full drive overwriting process are two separate things, and it is not safe to count on FDE to be equal to full drive overwriting, even though many implementations of FDE probably do a full drive overwrite as well. here is another citation, although from a non-official source: http://serverfault.com/questions/395912/does-luks-also-encrypt-free-space  Once again you can see that the actual FDE does not overwrite the entire drive, and relies on a separate (although depending on the specific implementation, possibly integrated) drive wiping mechanism.