Manually zeroize old data - Futurex Documentation

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.

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$ umount <partition name>
$ wipefs <partition name>
$ cat /dev/zero > <partition name>

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

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$ umount /dev/sdb
$ wipefs /dev/sdb
$ cat /dev/zero > /dev/sdb