I had a truecrypt encrypted partition with a hidden volume. Mistaking it for a backup drive, I created a new hidden volume ("direct mode" in the volume creation wizard). This process doesn't overwrite the encrypted data but it overwrites the header (and the backup header), so I'm guessing that recovering the data would be impossible. But I ask here just in case there's something I haven't considered. If it's of any relevance, the new volume had the exact same settings (password, size, algorithm) as the old one, but obviously the entropy would be different.