OS X 10.8.3: Fusion Possibilities For NonFusion Equipped Mac

We’ve been waiting and waiting for Apple to release the next version of OS X Mountain Lion in hopes that the next full version would have all the necessary components to setup a Fusion drive on any Mac capable of installing a hard drive and SSD together. A little over a week ago, Apple released OS X version 10.8.3 and, with one small caveat, our hopes were fulfilled.

The Profusion Of Fusion Confusion

But before we get to showing you exactly how to setup your own DIY Fusion drive, I’d like to dispel some mis-information that has been floating around the web. Up until now, most of the reports you’ve read about creating your own DIY Fusion drive on a machine have been incomplete. There have been many tutorials on how to create a Core Storage volume that have been labeled as “how to create a Fusion drive”. They are two similar, yet different drive configurations. I’ve addressed a lot of this information in comments on the OWC Blog, but figure it would be a good idea to review and further explain what a Fusion drive actually is as opposed to a Core Storage volume.

When Apple introduced Lion, they added a logical volume manager (Core Storage) to the OS. The key factor to Core Storage is that it allows a single volume to span multiple physical disks. Which makes the Fusion drive possible, but it isn’t the only aspect.

What turns a Core Storage drive into a Fusion drive is the introduction of automated storage tiering to mix. This has actually been around since 2005 on larger scale networks where the software moves data across different disk types and RAID levels in order to balance space, cost and performance requirements of a server. Prior to the automation software, this type of data manipulation was done manually.

Most of the terminal command setups we’ve seen online are only initiating that Core Storage volume. It needs the software to run the automated storage tiering to make it a true Fusion Drive. Until now, we’ve only seen that software component in the 2012 Mac mini and 2012 iMac models that ship with their specific builds of 10.8.2

You don’t find out that it’s not truly ‘Fusion’ until the SSD portion has been completely filled up. And even then – what’s on the SSD continues to be read at full SSD speed, so it’s only the new data writes (where existing data on SSD is not being replaced) and subsequent reads of that HDD stored data that are slower due to being on the HDD. The way a Core Storage volume works, it really makes people think they’ve created a true Fusion drive. So, now that you know the difference, the question on everyone’s mind is…

How Do I Create A True DIY Fusion Drive?

Items Needed:

  • A Mac that you can install both a Solid State Drive and a Hard Disk Drive into. So that’s the iMac (2009 or newer), MacBook (Late 2008 Aluminum, or Early 2009 or newer) with Data Doubler, or MacBook Pro (Late 2008 or newer) again with Data Doubler, Mac mini (Early / Late 2009, Mid 2010 Server, and Mid 2011 or newer) with Data Doubler Kit or Data Doubler where applicable, or Mac Pro (Early 2008 or newer).
  • A hard drive and an SSD installed/to install internally – Fusion is designed to work on internal drives only.
  • An external drive to clone to or a Time Machine backup drive – creating a Fusion volume will erase both the SSD and the hard drive, so if you have information on the hard drive you want to keep, you’ll need to have a copy of that data elsewhere.
  • An external hard drive or USB flash drive that is 8GB or larger to boot your  OS X 10.8.3 installer.

Step 1 – Have a copy of your computer’s data.

This process will erase both the installed SSD and hard drive, so if you have data on one or both of these drives, you’ll want to have a copy that’s not on either of the two drives that are going to be part of the Fusion array. If you are installing both a new SSD and a new platter-based drive into, you can put your original drive in an external enclosure, and your data will be there, out of the way. If you’re using the same drive that you already have installed, you will need to copy that drive’s contents to an external one.

Step 2 – Download 10.8.3 from the App Store.

And here’s the caveat I mentioned earlier: At this time, you absolutely need to download OS X 10.8.3 from the Mac App Store. To further clarify, we’re talking about the FULL VERSION – not the update. This is the only way to assure you have the correct version of Disk Utility on your recovery partition. You can do this by going to App Store > Purchases > Click the Download button to the right of OS X Mountain Lion. ***you may need to actually purchase ML if it does not show in the Purchases screen.

Step 3 – Move the Installer to your Desktop.

The Mountain Lion install package will show up in the Applications folder once downloaded. Move it to your desktop.

Step 4 – Right click on the package icon and select Show Package Contents.

Step 5 –  Navigate to Contents > Shared Support.

There you will see InstallESD.dmg. Drag this image file to your desktop.

Step 6 –  Using Disk Utility, restore the InstallESD.dmg file to an External Hard Drive or USB Flash Drive at least 8GB in capacity.

This should be a separate drive other than your clone or Time Machine backup.

Step 7 – Install the new drive(s) in the computer you’re upgrading.

See our video page for our step-by-step instructions on installing one or both drives into your machine.

Step 8 – Boot to the newly created 10.8.3 Installer.

Boot the machine while holding down the Option key at startup and selecting the external hard drive or USB flash drive with the 10.8.3 installer (it’ll have the orange icon).

Step 9 – Open Terminal.

If you installed at least one brand new drive, you will likely get a message about a disk being unreadable. That’s okay; just click “Ignore.”  We’ll be initializing it over the next couple of steps.

You can then open Terminal. You can find it in Applications/Utilities/Terminal.app

Step 10 – Find Your Disk IDs.

In Terminal, type: diskutil list

This will have the command-line version of Disk Utility (diskutil) that lists all the disks attached to your computer. In the results, you will find the disk IDs of the HDD and SSD. Take note of these ID numbers. In most cases (2 drives internally and booted from the external), the IDs will be “disk0” and “disk1.” However, individual results may vary, depending on your setup, so you’ll want to make sure you have the right drives.

Step 11 – Create the Fusion drive array.

In Terminal, type: diskutil cs create drivename driveIDs

This is the command that actually tells your Mac to tie the drives together in a Fusion array.

Broken down, the step does this:

  • diskutil – the command-line version of Disk Utility.
  • cs – this invokes Core Storage, which is necessary for Fusion.
  • create – creates a Core Storage group.
  • drivename – this is the name of the drive and how you want it to appear in Disk Utility (not the Finder – that comes later). You can call it whatever you want; in our example, we named our Fusion array “Fusion.”
  • driveIDs – these are the drive IDs of the drives you want as part of your Fusion array, separated by a space. In our example, they are “disk0” and “disk1”, but it may be different in your setup.

Once you enter in this command, it’ll do its thing and set-up the drives into an array for Fusion.

Step 12 – Get ID information for Fusion array.

In Terminal, type: diskutil cs list

This will give you a listing showing any Core Storage Logical Volume Groups (aka Fusion drive). You will need to do two things here. First, copy the long alphanumeric string for the Logical Volume Group, then note the Free Space for it. You will need both of these for the next step.

Step 13 – Format the Fusion drive so you can put files on it.

In Terminal, type: diskutil cs createVolume groupString jhfs+ volumeName size

This command creates a volume on the Fusion array where you can place your files. Again, since some important stuff is going on here, let’s break down the command.

  • diskutil – again, this is the command-line version of Disk Utility.
  • cs – invokes Core Storage functions, which are necessary for this arrangement.
  • createVolume – this is the command to create the actual storage area for the drive that is represented on your desktop by an icon.
  • groupstring – this is the long alphanumeric string you copied from the previous step. It identifies that the array you created as the one getting a volume placed on it.
  • jhfs+ – the format of the drive. This is Apple Extended Format (journaled), which is recommended for drives with an OS installed on it.
  • volumeName – the actual name of the volume, how it should appear underneath the icon. If there is a space in the name, you should either put the entire name in quotes (“Drive Name”) or put a backward slash before the space (Drive\  Name). In our example, we did the latter, naming our volume “OWC Fusion.”
  • size – this is the size of the volume. In our example, we had a 1.1TB drive. We used “1100g” to denote it as 1100GB (1.1TB in base 10). Alternatively, we could have also used 1.1T, or even 100% as a size.

Once you have this information entered, hit Return and let it do its thing; the Fusion Drive will then be available in the Finder.

Step 14 – Close Terminal and Install OS X.

Now that we have created the CoreStorage volume named Fusion, we can now install the OS and bring over your data.

Close your Terminal window and select the option to Install OS X. Follow the prompts for installation, choosing your new Fusion drive as the destination. You will need an Internet connection to do this; an Ethernet connection is preferable, though you will also be able to use an AirPort connection, albeit at slower speeds.

Step 15 – Migrate over your information.

As part of the setup for your new installation, you will be asked if you wish to import data from another disk; you will want to. Attach and select your clone or Time Machine backup and Migration Assistant will bring over your data.

Step 16 – Enjoy your new installation.

Once migration has completed, shut down your computer and disconnect your clone. At this point, you will have OS X running on a Fusion drive on your computer. You can now use it like you would any other drive.

Things to consider before committing to a Fusion setup

As with any drive setup, there are pros and cons to a Fusion array. The pros, as mentioned at the beginning of the article are that it appears single volume and works automatically to keep the best speed. However, there are a couple of cons that you should also be aware of.

You will need a backup.

While a backup plan for your computer is something you should have anyway, this becomes even more important for Fusion drive equipped Macs. The way Fusion is set up, if either the hard drive or the SSD fails, the data on both drives is lost. Having a reliable, frequent backup plan will be essential in protecting against data loss.

Performance may not be enough for high-end professional use.

Apple claims near-SSD performance for Fusion-equipped drives. For casual use (email, Web browsing, basic iPhoto use, etc.), this is largely true. From testing both in-house and by Lloyd Chambers of Mac Performance Guide, a Fusion drive will first fill the faster SSD portion, then start filling the slower hard drive. Once writing is complete, data will be moved from the SSD to the hard drive until there is 4GB free on the SSD again.

The trouble comes when you start working with larger files, such as with pro audio, video and large-scale photo work. Often, these files far surpass the 4GB size, so you will see fast SSD transfer speeds followed by a precipitous drop in speed when it transfers over to the hard drive. For a full rundown of testing, check out Lloyd’s writeup at Mac Performance Guide.

For those that a Fusion drive just isn’t going to be practical, you may be better served using a Hard Drive/SSD 2-drive setup with a relocated home folder. You reduce the risk of losing all your data at once, while still retaining a large portion of the speed/storage benefits of Fusion, but with more flexibility.


  • Hi thank you for this guide.
    Is the creation of a fusion drive also possible with (diskutil of) OSX versions later than 10.8.3 ? Can I follow the same steps with El Capitan ?

  • Excellent guide. Performed the steps as you outlined with Yosemite and went without a hitch. I was wondering if you have a guide on how to upgrade the SSD portion to a larger size. For example I start with a 128GB SSD + 2TB HDD today and next year want to Upgrade to 512GB SSD + 2TB HDD. Is this even possible with out starting from scratch?

  • The Mac OS 10.10.5 update destabilized my DIY Fusion Drive : (
    Disk Warrior now sees the drive as: Incompatible Format.

  • Excellent guide! Only hiccup I encountered was booting from flash drive with 10.8.3 on it. I followed the above steps exactly, but my MBP would not recognize the flash drive as an option. Unable to resolve this, I simply booted from the external drive with the recovery and time machine on it and everything worked out perfectly.
    The results are incredible! This solution fit my needs for high speed, high storage capacity, and (relatively) low cost. I am VERY pleased and highly recommend this. If you are unfamiliar with command line, and haven’t used terminal before don’t sweat it. The guide doesn’t make any assumptions and tells you everything you need to know. Thanks OWC!