GUIDE: DIY Fusion Drive Guide

1.1. Hybrid Drive & Fusion Drive: What’s the difference?

In a typical hybrid drive like the Seagate Momentus XT, fast flash memory speeds up performance while a standard 7200-rpm platter-based drive holds the data. (The 1TB Momentus XT has 8GB of NAND flash.) A hybrid drive looks and acts like a regular single hard drive.

A Fusion Drive consists of two separate drives—one hard drive and one solid-state drive—that are “fused” together. Apple uses software to create a single volume out of the two drives, but the result is NOT a RAID array. The user, and the applications on the Mac, see the Fusion Drive as a single drive.​

1.2. Purpose of a Fusion Drive?
Many people who want both speed and capacity already use a combination of solid-state drives and hard-disk drives; but usually these drives are two separate volumes, and trying to figure out where you should put your files and apps can be complicated to some. Fusion Drive technology takes care of the housekeeping for you, letting you spend more time working and less time organizing.​

1.3. What does a Fusion Drive actually do?
When you purchase a Mac with an optional Fusion Drive (currently, a Fusion Drive is a $250 upgrade to the $799 Mac mini), the OS and all of the applications that ship with a new Mac are loaded onto the flash portion of the Fusion Drive. Again, the user can’t tell that there are two drives, or where a particular file or application is stored, and 128GB (the size of the SSD in the Mac mini's Fusion Drive) is plenty of room for the OS and programs.

The software that controls the Fusion Drive looks for data that you frequently use. If you access certain documents a couple of times, the software could identify those files as something to keep on the SSD. The Fusion Drive knows the difference between user access and system access to files; and when it decides on what to move to the SSD, the Fusion Drive watches for files that the user is accessing. For example, the Fusion Drive will not move files that Time Machine has touched.

The Fusion Drive fills up the SSD; later on, data that you haven’t used in a while moves to the hard drive when the computer is idle. A portion of the SSD—at least 4GB—is set aside so that all data writes to the SSD first while you are working, unless you move more than 4GB at a time.​

1.4. How does a Fusion Drive move data?
A Fusion Drive moves data in blocks, not file by file. It’s possible that pieces of a file could reside on both disks. The Fusion Drive doesn’t cache copies of files onto the SSD portion; it moves the data that needs the fastest access directly to the flash section.

Fusion Drive technology isn’t magic, though. If you fill up the SSD, the process of copying very large files will at some point slow down sharply, matching the relatively poky speed of a standard spinning-platter hard drive.

For what most of us do most of the time, however, a Fusion Drive works great. It performs like a regular SSD—in other words, fast. A high-end, 2.3GHz quad-core Core i7 Mac mini with Fusion Drive and 4GB of RAM was more than three times as fast in our file copy and unzip tests as the same Mac mini with a standard 1TB 5400-rpm drive. The standard drive took nearly twice as long to import 500 pictures into iPhoto, while the Fusion Drive was 30 percent faster than the standard drive in our Photoshop and Aperture tests.​

1.5. Can I build my own Fusion Drive?
When we opened the Mac mini with Fusion Drive, we tried to tinker with all sorts of things. Getting to the RAM is very easy: You simply turn the round rubber base on the bottom a half inch clockwise to take it off and expose the two RAM slots. The situation gets trickier the deeper you go inside the Mac mini, however, and getting to the second drive requires pulling just about everything out of the system.

When we had the drives in hand, we found that we could connect one externally to another Mac running OS X 10.8.2. Disk Utility saw the drive but spelled its name out in ominous red letters, asking if we wanted to repair the drive. We clicked No. We then connected the second drive externally—and lo and behold, the Fusion Drive mounted, and we could copy files back and forth.
DIY: You can use the command-line tools for CoreStorage to build your own Fusion Drive.

We were also able to create our own Fusion Drive using the CoreStorage tools found in the command-line verion of Disk Utility. Those tools also let you change the drives from CoreStorage volumes to standard drives. The HFS+ formatting lies on top of the CoreStorage layer. I don’t know why anyone would want to separate the drives from their Fusion Drive (a family of CoreStorage volume devices), but it can be done. The OS remembers that the drives were once fused together, so Disk Utility will see them as in need of repair until they are reformatted.

Apple does a lot of testing, and carefully chooses the mechanisms for the Fusion Drive. When rolling your own Fusion Drive, you need to back up your data: As with a striped RAID 0 volume, if one drive goes south, all of your data is lost. You'll find no redundancy in a Fusion Drive.
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You need two (2) drives to make a Fusion Drive, and if you want to get the speed boost, one should be an SSD.
Apple chooses the drives they use based on extensive testing, which is why they recommend buying a Fusion Drive at the point of purchasing your Mac. Apple does not support users who have built their own Fusion Drive. (In our lab experiments, I was able to create a Fusion Drive with two flash thumb drives.)​

If you have anything that you want to keep on either of the drives, now is the time to back it up: linking the drives with CoreStorage, the technology behind the Fusion Drive, will erase them completely. If you need help with backing up your data, check out our guide on how to back up your data with Time Machine or, if you don't need to save absolutely everything on your drive, our more generic guide on backing up your data.​

The Disk Utility application (Application > Utilities > Disk Utility) doesn't support the management or creation of Fusion Drives in its graphical interface, and knowing Apple, it might not ever support it. To create the Fusion Drive, we'll use Disk Utility, but the command line version that comes with every Mac.

If you aren't going to include the drive you booted from in a Fusion Drive, you can open the Terminal app (Applications > Utilities > Terminal). This would be the case, say, if you're using a Mac Pro with multiple drive bays, since you can format them both externally.

If you want to include your current boot drive as part of the Fusion Drive, you're going to need to boot into recovery mode and run Terminal from there. The CoreStorage process used to "fuse" the drives also formats them, and you can't do that to a drive used as the boot drive. You can boot into recovery mode by holding Command+R when starting up your Mac, and open Terminal from there.

One last thing to note: If you're using a Mac currently in Apple's product line, such as the 13-inch Retina MacBook Pro or the new Mac mini, you can't use just any old install disc as a boot disc, you will need a bootable USB Thumb Drive with OS X 10.8.x or use Internet Recovery.

Once you have the Terminal open, you're ready to get to creating the Fusion Drive.

List the drives you system can see. Use the following command to list the drives attached to your system:
This will list the drives like this:


The drive mount points are labeled /dev/disk#. Make a note of the mount points for the disks you want to make into a Fusion Drive. In our example, /dev/disk0 is the boot disk, while /dev/disk1 and /dev/disk2 are the SSD and hard drive (respectively) that we want to put together as a Fusion Drive. An easy way to tell drives apart is by their size and their name. Make sure you don't confuse them, as the Terminal doesn't give much warning before wiping your drive.

Create a logical volume group: Create a CoreStorage logical volume group, the pool of data that will be made from the combined space of our physical drives. Use the following format of the diskutil command:


Going off the example using /dev/disk1 and /dev/disk2 as the drives to combine, I simply type:

When the process is done, your logical volume group should be completed and the command will finish by presenting you with a unique identifier for the group, which you should copy to your clipboard. It should look similar to this:



Create a logical volume: Now that we have a logical volume group, we can create the logical volume, what your Mac will recognize as a single drive. Use the following format of the diskutil command:

• lvgUUID is the unique identifier you copied from the previous step.
• type use Journaled HFS+ (typed as jhfs+).
• name can be whatever you want to name the drive (traditionally "Macintosh HD"). Make sure you use quotes if there's a space in the name.
• size is how much of the "pool" that you want to make into a drive, using the following suffixes: B(ytes), S(512-byte-blocks), K(ilobytes), M(egabytes), G(igabytes), T(erabytes), P(etabytes), or (%) a percentage of the current size of the logical volume group.

Going off my previous example, my command would look like this:




That will create a volume named Macintosh HD, and fill the entire logical volume group (100%), or all the space on the two drives.

Once that command is done, you have your Fusion Drive. You can now restore Mountain Lion back onto it, or do whatever you want. The only thing to remember is that they both need to be connected at the same time to work.
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Apple's Fusion Drive combines a fast solid state drive with a slow, platter-based hard drive to deliver both speed and capacity. It does this well, considering it requires no interaction from the user. However, you may want to separate a Fusion Drive, either to gain a greater level of control or to simply swap out one of the drives. Whatever your reasons are, separating a Fusion Drive is a fairly simple process.

Backup Data: Splitting up a Fusion Drive will erase any data contained on it. if you want to preserve any data, back up first. You can check out our guide to backing up with Time Machine or our more general guide if you don't need to save everything.

If the two drives you want to separate aren't currently running the OS but are attached to your computer, you can just open Terminal (Applications > Utilities > Terminal). Perhaps you created a Fusion Drive with a pair of external drives, or you have a Mac Pro, which has multiple drive bays, and you've paired a couple of drives in the bays as a Fusion Drive (but aren't used as a boot drive).

If you're currently running OS X on the Fusion Drive , you must run the Terminal from Recovery Mode. You can get there by holding Command+R while booting. If you have only two drives in your computer configured as a Fusion Drive, then it will also be used as the boot drive and you need to boot into Recovery Mode.

Splitting Drives: To split the drives, you need to delete the CoreStorage logical volume group, which is the group of physical drives that make up your Fusion Drive. First, you need to find the unique identifier for the logical volume group. You can do this by typing the following command into the Terminal:



You should see a table similar to the one above. At the top, you'll see the logical volume group and its unique identifier just to the right of it, it's a long hyphenated string of numbers and letters. Copy that and then use the following command to separate the Fusion Drive (replace lvgUUID with the identifier you copied):



When you destroy the CoreStorage Logical Volume Group, it formats the drives for you as normal Mac OS Extended (Journaled) volumes (JHFS+). This is the normal type of volume that you would install OS X on or use to format an external drive.

Fusion Drive Quirks: If you split up a Fusion Drive you got from Apple, you won't be able to use Disk Utility to format the drives. Disk Utility will detect that the two drives used to be an Apple Fusion Drive and show them in red text.

Not to worry though, as you can install Mountain Lion without a problem. Just exit Terminal after you delete the CoreStorage setup, and choose to reinstall Mountain Lion from the list of choices in Recovery Mode. You can select which drive to install the OS on, and upon booting you'll have access to both your drives. You'll also be able to go into Disk Utility and format them once you've installed Mountain Lion, though if you boot from Recovery Mode you'll still get the same problem formatting them.

Recovery Partition: Splitting up a Fusion Drive eliminates the recovery partition, so make sure you have access to the Internet in order to download the latest version of Mountain Lion onto your machine. The late 2012 Macs that shipped with Fusion Drives require a special version of Mountain Lion not yet available anywhere but by Internet Recovery.

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