Setting Up Raid iMac
As of El Capitan, the Disk Utility application no longer provides RAID functionality, but it remains in the diskutil command-line program. For brief instructions, type diskutil appleRAID or diskutil appleRAID create in the terminal. For more detailed information, type man diskutil. 

List of available RAID commands:

askewchan ~&gt; diskutil appleRAID
Usage:  diskutil [quiet] appleRAID|AR &lt;raidverb&gt; &lt;options&gt;,
        where &lt;raidverb&gt; is as follows:

     list            (Display the current status of RAID sets)
     create          (Create a RAID set on multiple disks)
     delete          (Delete an existing RAID set)
     repairMirror    (Repair a damaged RAID mirror set)
     add             (Add a spare or member disk to an existing RAID)
     remove          (Remove a spare or member disk from an existing RAID)
     enable          (Convert a volume into a single disk RAID set)
     update          (Update the settings of an existing RAID)

diskutil appleRAID &lt;raidverb&gt; with no options will provide help on that verb


The create command:

askewchan ~&gt; diskutil appleRAID create
Usage:  diskutil appleRAID create mirror|stripe|concat setName
        filesystemType MemberDeviceNames|NestedRAIDSetUUIDs ...
Create a RAID set.  All existing data on the drives will be lost.
Ownership of the affected disks is required.
Example: diskutil AppleRAID create mirror MirrorName JHFS+ disk1 disk2


Some relevant parts of the full man page:

askewchan ~&gt; man diskutil

DISKUTIL(8)               BSD System Manager's Manual              DISKUTIL(8)

NAME
    diskutil -- modify, verify and repair local disks

SYNOPSIS
    diskutil [quiet] verb [options]

DESCRIPTION
    diskutil manipulates the structure of local disks.  It provides information
    about, and allows the administration of, the partitioning schemes, layouts, and
    formats of disks. This includes hard disks, solid state disks, optical discs,
    CoreStorage volumes, and AppleRAID sets.  It generally manipulates whole vol-
    umes instead of individual files and directories.


On RAID:

appleRAID | ar raidVerb [...]
    AppleRAID verbs can be used to create, manipulate and destroy
    AppleRAID volumes (Software RAID).  AppleRAID supports three basic
    types of RAID sets:

          o   "stripe" - Striped Volume (RAID 0)
          o   "mirror" - Mirrored Volume (RAID 1)
          o   "concat" - Concatenated Volume (Spanning)

    Of these three basic types, only the "mirror" type increases fault-
    tolerance.  Mirrors may have more than two disks to further increase
    their fault-tolerance.  Striped and concatentated volumes are, in
    fact, more vulnerable to faults than single disk volumes.

    AppleRAID is not a replacement for backing up your data.  Backups
    should be always be performed on a regular basis and before modify-
    ing any RAID set using these commands.


On creating a new RAID:

    create mirror | stripe | concat setName format devices ...
               Create a new RAID set consisting of multiple disks and/or
               RAID sets.  setName is used for both the name of the cre-
               ated RAID volume and the RAID set itself (as displayed in
               list). e.g. 'diskutil createRAID stripe MyArray JHFS+
               disk1 disk2 disk3 disk4'.  Ownership of the affected
               disks is required.  diskutil createRAID is a deprecated
               synonym for diskutil appleRAID create.