The next step is to use the disk names we looked up on page 2 of this guide to assign the drives to a logical volume group that core storage can use.
Create the Logical Volume Group
With the disk names at hand, we're ready to perform the first step in creating a Fusion drive, which is creating the logical volume group. Once again, we will use Terminal to execute the special core storage commands.
Warning: The process of creating the logical volume group will erase all data on the two drives. Be sure to have a current backup of the data on both drives before you begin. Also, pay special attention to the device names you use. They must exactly match the name of the drives you intend to use in your Fusion drive.
The command format is:
diskutil cs create lvgName device1 device2
lvgName is the name you assign to the logical volume group you are about to create. This name won't show up on your Mac as the volume name for the finished Fusion drive. You can use any name you like; I suggest using lowercase letters or numbers, with no spaces or special characters.
Device1 and device2 are the disk names that you wrote down earlier. Device1 must be the faster of the two devices. In our example, device1 is the SSD and device2 is the platter-based drive. As far as I can tell, core storage doesn't do any type of checking to see which is the faster device; it uses the order you place the drives in when you create the logical volume group to determine which drive is the primary (faster) drive.
The command for my example would look like this:
diskutil cs create fusion disk0s2 disk1s2
Enter the above command in Terminal, but be sure to use your own lvgName and your own disk names.
Press enter or return.
Terminal will provide information about the process of converting your two drives to members of a core storage logical volume group. When the process is complete, Terminal will tell you the UUID (Universal Unique Identifier) of the core storage logical volume group it created. The UUID is used in the next core storage command, which creates the actual Fusion volume, so be sure to write it down. Here is an example of the Terminal output:
CaseyTNG:~ tnelson$ diskutil cs create Fusion disk0s2 disk5s2
Started CoreStorage operation
Touching partition type on disk0s2
Adding disk0s2 to Logical Volume Group
Touching partition type on disk5s2
Adding disk3s2 to Logical Volume Group
Creating Core Storage Logical Volume Group
Switching disk0s2 to Core Storage
Switching disk3s2 to Core Storage
Waiting for Logical Volume Group to appear
Discovered new Logical Volume Group "DBFEB690-107B-4EA6-905B-2971D10F5B53"
Core Storage LVG UUID: DBFEB690-107B-4EA6-905B-2971D10F5B53
Finished CoreStorage operation
Notice the UUID that was generated: DBFEB690-107B-4EA6-905B-2971D10F5B53. That's quite an identifier, definitely unique and definitely not brief and memorable. Be sure to write it down, because we will be using it in the next step.