Manually zeroize old data

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

Use the unmount and wipefs commands to unmount and zeroize partitions:

For example, to zeroize partition /dev/sdb: