The graphical interface of Mac Disk Utility in Mac OS 10.11 (El Capitan) does not include the ability to create a RAID. However, the Mac Terminal can still create a RAID with simple text commands.
IMPORTANT! Before proceeding, please safely eject and then physically disconnect all external storage devices (external hard drives, SD cards, etc) from your Mac computer, except the device on which you want to create the RAID.
IMPORTANT! Do not attempt the following while your Mac is in Recovery Mode. Proceed only when your computer has started normally.
If you want to create a software RAID in Mac OS 10.11, please follow these instructions:
From the Desktop, select Go from the top menu bar, then Utilities > Terminal.
Type the following:
A list of all storage devices connected to or inside your computer will appear.
Find and note the external, physical drives that you want to include in the RAID.
E.g., disk0 and disk4
Decide which RAID mode you want - striped (RAID 0) or mirrored (RAID 1).
Type the following for RAID 0:
diskutil appleraid create stripe [Array Name] JHFS+ disk_ disk_
For example: diskutil appleraid create stripe LaCieRAID0 JHFS+ disk0 disk4
Type the following for RAID 1:
diskutil appleraid create mirror [Array Name] JHFS+ disk_ disk_
For example: diskutil appleraid create mirror LaCieRAID1 JHFS+ disk0 disk4
Then press Enter.
The Disk Utility will create the RAID.
The RAID volume is now mounted and ready to use.