Manually zeroize old data

For each partition where you are not preserving existing data (where encrypted_preserve=n) and which previously held sensitive data, you should wipe the partition before being encrypted. Otherwise, some blocks might not be overwritten with encrypted data immediately and could be recovered.

Use the umount and wipefs commands to unmount and zeroize.

For example, to zeroize partition /dev/sdb, run the following commands: