GUI ではゼロ消去できないのでターミナルを開いて…
Usage: diskutil zeroDisk [force] MountPoint|DiskIdentifier|DeviceNode
Write zeros across the given whole disk or partition. After this operation, a whole disk will need to be partitioned; a partition will need to be erased (newfs'd), in order to be used again as a volume. Specifying "force" will do best-effort force unmounts and shared writes. Ownership of the affected disk is required.
他にも El Capitan について諸々がこちらに。
新・OS X ハッキング! (146) El Capitan」のココが気になる ～Disk Utility編～ | マイナビニュース