Create Raid 1
diskutil createRAID mirror MirroredVolume JHFS+ disk1 disk2
diskutil createRAID mirror MirroredVolume JHFS+ /Volumes/Server\ HD /Volumes/Server\ HD\ 2/
RamDisk
This will create a disk of Memory (Ram) which will be fast access. Remember you can't use it for data storage.
diskutil erasevolume HFS+ "ramdisk" `hdiutil attach -nomount ram://1165430`
