:) Voici ce que me donne la commande :)
-bash-3.2# diskutil eraseVolume jhfs+ "Macintosh HD" disk20
Started erase on disk20 OS X Base System
Unmounting disk
Erasing
Initialized /dev/rdisk20 as a 465 GB case-insensitive HFS Plus volume with a 40960k journal
Mounting disk
Finished erase on disk20...