[iuybuyvvyu]

But everytime you write to it every byte in the volume will change and dropbox will have to resync the whole file.

[mbrubeck]

Is that true? If I use TrueCrypt to encrypt my entire 200GB hard drive, does it have to update every block on the disk when I write to a single file?

[Gormo]

Some of the info in the TrueCrypt FAQ (http://www.truecrypt.org/faq) leads me to believe that each block is independently encrypted.

See question "What will happen when a part of a TrueCrypt volume becomes corrupted?"

[iuybuyvvyu]

Ideally it should otherwise you are vulnerable to a range of differential attacks. In practice it's a compromise between performance and security.

It might very well touch a large number of blocks to stop an attacker working out where on a disk a particular file is. There are a bunch of rsync freindly crypto implementations that minimize this effect

[Devilboy]

How exactly would an attacker have enough access to your encrypted data to do a diff attack? If he has physical access to the machine you're going to be compromised regardless of the details of the encryption scheme.