HOW TO Set up a Fusion Drive
1 Preparatory measures
Back up everything, and detach all external drives you don’t want to use in the Fusion Drive; any volume used will be erased. Connect your SSD and open Terminal; if you’ll add your startup disk, start up in Recovery (hold ç+R) first.
4 Create a volume
The combined drive looks to OS X like a regular blank one, so you need to create a volume on it by entering the following, replacing xxx with what you just copied: diskutil cs createVolume xxx jhfs+ "Macintosh HD" 100%.
2 Identify components
Enter diskutil list to identify the volumes to use. Though they’re easy to discern in Finder, you’ll need to refer to them by what’s listed under Identifier. For us, that’s /dev/disk0s2 (our startup disk) and /dev/disk2s2 (our SSD).
5 About that command
The previous command set the new volume to occupy 100% of the available space, erased it with an OS X Extended (Journaled) file system, and named it Macintosh HD. The combined drives now appear as one in Finder.
3 Bind the two parts To create a merged storage device, enter: diskutil cs create BootDrive
/dev/disk1 /dev/disk2. Disk Utility applies the required settings and gives it a Logical Volume Group Name – copy that to the Clipboard for the next step.
6 Install or restore OS X
You can now install El Capitan on your new Fusion Drive (or restore your Time Machine backup to it). OS X will manage the process of ensuring the most used data is kept on the SSD and other stuff goes on the hard disk.