How to Create a software RAID in Mac OS 10.11
Launch Terminal
In the Terminal.app, type the following:
diskutil list
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).
In the Terminal.app, type the following: for RAID 0:
diskutil appleraid create stripe [Array Name] JHFS+ disk_ disk_
For example: diskutil appleraid create stripe YOUR-RAID-NAME JHFS+ disk0 disk4
or
In the Terminal.app, type the following:for RAID 1:
diskutil appleraid create mirror [Array Name] JHFS+ disk_ disk_
For example: diskutil appleraid create mirror YOUR-RAID-NAME JHFS+ disk0 disk4
Then press Enter. The Disk Utility will create the RAID.