D'acc, voici le retour:
-bash-3.2# diskutil eraseVolume jhfs+ "Macintosh HD" disk20
Started erase on disk20 Macintosh HD
Unmounting disk
Erasing
Initialized /dev/rdisk20 as a 697 GB case-insensitive HFS Plus volume with a 57344k journal
Mounting disk
Finished erase on disk20 Macintosh HD...