Yes, only parts of the underlying file are changed. TrueCrypt uses XTS mode of operation (https://en.wikipedia.org/wiki/Disk_encryption_theory https://en.wikipe

Yes, only parts of the underlying file are changed. TrueCrypt uses XTS mode of operation (https://en.wikipedia.org/wiki/Disk_encryption_theory https://en.wikipedia.org/wiki/Disk_encryption_theory, http://www.truecrypt.org/docs/modes-of-operation http://www.truecrypt.org/docs/modes-of-operation).