Fusion Drive on an 27-inch, Mid 2011 iMac

Recently, Apple has begun to sell iMacs and Mac Minis with Fusion drives. Two drives are combined: a small fast, solid state drive and a large, slower hard disk. The unification is done in software, through a storage layer that prioritises writing operations and heavily used files to the SSD and actively shuffles less used files onto the slower disk. It’s baked into Mac OSX.

When I bought my desktop iMac at the beginning of 2012, I wanted to ensure I had sufficient storage for future projects. At this time, Fusion drive wasn’t available in Macs (but was implemented as a part of CoreStorage in Lion, unbeknownst to many). I bought a hybrid system with a 256Gb SSD and a 1TB HD. This arrangement of internal drives is good for an implementation of a Fusion drive.

My main motivation for reconfiguring the two drives may seem odd- the second slow HD was always going to sleep due to lack of use! This sleep meant that new instances of Finder windows, following a period of disk inactivity, took some seconds to jump to life, lagging a whirring noise as the HD woke from sleep to be presented in the sidebar. I wanted to unify the drives to stop this happening. Also, I was keen to pursue the main objective of the Fusion drive: get a single large fast logical drive that would be comparatively easier to organise. I decided to go ahead, even though I anticipated problems and potential failure, after reading some posts.

What did I begin with: I made a Carbon Copy Cloner image of the boot SSD on a Firewire drive. I also backed up the recovery partition (again with CCC). I booted into this copied recovery partition on the FW drive (hold down “option / alt” during startup) so that I had full access and free rein over both my internal drives. I also had recent Time Machine backups. In this way, I had two options for restoring my original configuration. I also copied the (much less important) contents of my Macintosh HD2 onto the Firewire drive too.

Creating the core storage fusion volume in the Terminal under the recovery console is very easy, and takes seconds. “cs” is for CoreStorage. This action does involve vaporising all the contents of both drives though. You have been warned!

diskutil cs create FusionDrive disk0 disk1

You need to run the “list” command to get the LVG-UUID. The LVG-UUID will be a long hex code like: F663F929-3bdf-4DAD-99EF-FFFF1111FFFF

diskutil cs list
diskutil cs createVolume [LVG-UUID] jhfs+ MacintoshFD 1150g

Detailed instructions are here. I have no qualms about using the Terminal. Time Machine backups give you a lot of confidence in this respect. You can choose any name (‘MacintoshFD’).

Interestingly, what I thought might be a clever shortcut (restoring from a Time Machine backup) failed because the Lion recovery partition could not be created. In case of problems, there are some detailed tips here about how to get the recovery partition created. I knew that this step was a potential stumbling point – it also goes wrong when you try to update to Lion (and possibly Mountain Lion) on a disk that has a Boot Camp Windows partition.

I didn’t have to monkey about with the format of the volume (JHFS etc) under a side-boot in order to get the recovery partition as some have reported – instead I just asked for a reinstall of Mountain Lion over the wire from Apple. This download/install ran over a weekend – so I don’t know exactly how long it took.

Following a Time Machine restore (~150Gb, about 3h over Ethernet), I logged in, and began to work more or less as normal. I now have a 1.2TB fusion drive, and no Finder lag.

Surprisingly, some settings didn’t carry through which is quite unusual for Time Machine restore. The printer gave me a little exclamation mark and told me that some of software was missing. Single click to download it afresh. You might have a more stubborn printer. Uninstall followed by reinstall worked for me in another case.

The name of my computer (“Wyman” –Jeffries not Bill) didn’t make it, and the name I designated for the core storage volume “Macintosh FD” was reverted to the default “Macintosh HD”. A quick diversion into the Sharing panel of the System Preferences fixed the former. A right click to the drive name allowed me to fix the latter.

Time Machine also got confused, but could easily be righted by deselecting and reselecting the backup disk.


After two weeks – no problems whatsoever. It’s almost boring. But Finder (and other) performance is snappy and a single disk is marginally easier to organise.


Highly recommended, if you have the right internal drives.


2 thoughts on “Fusion Drive on an 27-inch, Mid 2011 iMac

  1. I have the same set up and I can’t search my Outlook emails anymore with Spotlight or ‘search this folder’ in Outlook. It will only show files I added after the upgrade.

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