Com um pouco mais de pesquisa:
man diskutil - Modifica, verifica e repara discos locais
secureErase [freespace] level device
Securely erase a disk or freespace on a mounted volume.
Level should be one of the following
1 - Single pass randomly erase the disk.
2 - US DoD 7 pass secure erase.
3 - Gutmann algorithm 35 pass secure erase. Ownership of the affected disk is required.