Information
Certain Apple devices have 2 types of drives in them for faster operating system functions and software running. They are usually a smaller SSD for the OS and Apps and a larger standard drive for data (documents) storage. A Fusion Drive makes 2 drives to act as one.
Environment
Process
Creating a DIY Fusion Drive
- Boot the computer using an OS X USB Recovery Drive.
- Once the recovery partition has loaded, go to \Applications\Utilities\Terminal.app (or from the menu bar, Utilities | Terminal) to load the Terminal.
- The first command to enter will retrieve a listing of the drives connected to the Mac. Due to the destructive nature of the process, any data stored on the drives will be destroyed, so please make sure to backup anything on those drives you don't wish to lose:
diskutil list
- Each drive listed will contain a mount point in the following format:
/dev/disk#
Where the "#" designates a number assigned to the drive by OS X. Locate the mount points for the SSD and HDD you wish to use to create the Fusion Drive. They'll be needed in the next command.
- The second command will create the logical volume group using Core Storage, which will act as a container for the drives being pooled together. The command will require the exact drive mount points obtained in step #4:
diskutil coreStorage create LOGICAL_VOL_GROUP_NAME DRIVE_1 DRIVE_2
For the "LOGICAL_VOL_GROUP_NAME," enter a name that will help define the storage group being created. DRIVE_1 and DRIVE_2 represent where to insert the drive mount points for the devices being fused. For example, if the group name was "FUSE" and the mount points were SSD = /dev/disk1; HDD = /dev/disk2, the command would be executed as follows:
diskutil coreStorage create FUSE /dev/disk1 /dev/disk2
*Please note that you can name the group anything, but it should be obvious its a Fusion Drive. i.e. Fusion. AND the SSD should be listed first as it is the smaller of the two drives.
- The third and final command needed in the Fusion Drive creation process will create the logical volume, which will actually be where data is stored once the process has been completed. The command will require the LVG UUID, a unique identifier that's assigned by OS X to all storage groups. Upon successful completed of the second command in step #5, the ID will be displayed as "Core Storage LVG UUID." Record the ID, as it will be necessary for the final command:
diskutil coreStorage createVolume lvgUUID type name size
i.e. diskutil coreStorage createVolume S87D6F8F-D9WJ-8AD9-SD88-VU89JI4FUI09 jhfs+ "Macintosh HD" 100% (Your unique ID alphanumeric string will be different, but you can copy and paste yours from the line above in Terminal.)
- Once the commands have been executed successfully, the Fusion Drive will have been created and the volume should now be mounted (and viewable) from within Disk Utility. If using the newly created drive as a boot drive, continue to install OS X as you normally would, making sure to select the Fusion Drive as the installation drive.
Disabling a DIY Fusion Drive
While disabling a Fusion Drive from Core Storage is not likely to be something done very often, you may find yourself in a situation in which it is necessary, such as upgrading the respective SSD and/or HDD drives or replacing a faulty drive.
To do so, simply execute this one command from Terminal:
diskutil coreStorage delete lvgUUID
To find the above UUIDm enter the following command from Terminal:
diskutil coreStorage list
As listed above, the "lvgUUID" is the unique identifier assigned to the volume. This can be obtained by executing a diskutil list. Once the lvgUUID for the Fusion Drive is located, execute the command, and the fuse will be broken.
Remember that prior to working on any storage related task, it's extremely importance to backup you data from the drive(s) you'll be working on. The process to both create or break a Fusion Drive is destructive, so any/all data contained in those drives will surely be lost.
Once the drives have been separated, booting from a USB recovery drive will once again allow you to reinstall OS X... or perhaps upgrade to a larger, higher performing Fusion Drive.