Dec 25

The new Mac Minis have super fast SSD Drives. I measured read times of 2500MB/s which is 5 times faster than the SSD drive in my old mac mini. But these drives are super super expensive. (An extra US$1400 for a 2TB drive). So I thought it might be an option to buy a Mac Mini with a small internal drive (256GB) and add an external SSD drive and combine them into a fusion drive. It didn’t work! But I thought I ‘d write a post in case anyone else has the same idea.

Disk Speeds

Here are the read speeds of various drive configurations measured with Black Magic disk test:

Old Mac Mini (2012):

Internal SSD drive: 480Mb/s:

New Mac Mini T2 2018:

Internal SSD: 2500Mb/s

External (USB-C) SSD: 530Mb/s

The new Mac Minis are much faster, but I was interested in the speed difference between the internal SSD and external SSD. It is so significant that this would be an ideal situation to make a fusion drive. A fusion drive was designed by Apple to ‘fuse’ an SSD drive to a slower spinning drive to ‘speed it up.’ The writes are done to the SSD, and the most used files are kept on the SSD. Then files are copied between the SSD and the slower drive in the background. So imagine being able to make an Fusion drive with a fast and a slow SSD.

Making a Fusion Drive.

Here’s how I made a fusion Drive:

Firstly I had to boot from the internet recovery partition. (Reboot and hold down Apple-R).

I plugged in my Samsung T5 drive and combined with the internal SSD drive of the Mac Mini to form a new Fusion Drive. The Fusion Drive is blank so you then need to reinstall OS X onto it. The instructions are here:

and here:

In summary, firstly you need to run this command below to find out the names of your drives:

diskutil list

Then you combine both drives into one substituting the numbers that you found above :

diskutil coreStorage create FUSION /dev/disk1 /dev/disk2

Then you need to create the drive volume:

diskutil cs createVolume logicalvolumegroup jhfs+ Macintosh\ HD 100%

After that you just format the drive as APFS, and install OSX. This gives the Fusion Drive a recovery partition and everything ready to go.

What worked, and what didn’t.

Well the drive seemed to work. The Internal Mac Mini SSD drive speed was 2500MB/s. The Samsung T5 SSD was 530MB/s. My newly created fusion drive was 2700!

This is the original internal SSD from the new Mac Mini:

Internal SSD
External Samsung T5 SSD
Fusion drive made with Internal SSD and External SSD.

Not only is the Fusion drive as fast as the Internal SSD (which is what I would expect), it’s actually FASTER!!! The fusion drive must be reading and writing from BOTH drives simultaneously. They had plenty of free space so this makes sense.

So what didn’t work? Well the computer was completely unstable. Random restarts. Crashing halfway through installing OS X. Disk Utility crashing. There was some crazy deep level conflict that meant the Mac just wouldn’t run properly! I rang Apple support and they talked me through splitting and re-makign the fusion drive but it was still unstable.

So in the end I had to split my fusion drive back into two separate drives.

If you use the Mojave command “diskutil resetFusion” to try to fuse 2 SSD drives it says that you can only create a Fusion drove from an SSD and a HDD.


With PCIe hard drives been so expensive it makes complete sense for Apple to allow Fusion drives made with a PCIe SSD combined with slower cheaper SSD. I’m not sure why this didn’t work. IF you try to make a fusion drive in Mojave with the Apple say they only support fusion of an SSD with a Hard disk drive.

I’d love to hear if anyone else has had success with this. Leave message below if you do!

UPDATE 13th Jan 2018: Yesterday I tried making a fusion drive from the internal Mac Mini drive and an external Seagate FAST SSD drive. It worked flawlessly installing Mac OSX up until the point where I rebooted, then it went into an endless repeating grey screen loop and never got to the desktop!

9 Responses to “Fusion drive on OSX with two SSD drives”

  1. nicos says:

    Thanks, very useful post. I have made a fusion drive with an external SSD and internal slow HD on my current iMac, and it’s been working very well.
    I was thinking to do the set up you’re describing here with the Mac Mini I’m about to get, so it’s good to know that fusing 2 SSDs create problems…
    So I guess we have to live with 2 drives on the desktop, and the added complications for Time Machine back up?

  2. Salomon says:

    Did you ever resolve this?

    I did something similar (SSD + internal NVME in iMac 5K) on Mojave (created is as HSF and let Mojave convert to APFS).

    It’s running well for me, stable. No issues. However – I’m not getting the NVME speeds in disk test. It’s being limited to SATA speeds.

    I found someone else that has a similar issue, but no solution so far.

    • Wayne says:

      No I could not get it working.

    • Xiaosong says:

      Hi Salomon,

      I’ve been running with the same issue earlier today, I am using a 1TB 970 eco to create a fusion drive with 128GB apple’s NVME SSD. I use the command above to create fusion drive and got SATA speed with is 500MB~ish speed.

      I recall that there are multiple drive names showing up when you submit `diskutil list` for each physical hard drive, should be look like this:

      /dev/disk0 (internal, physical):
      0: GUID_partition_scheme *121.3 GB disk0
      1: EFI EFI 209.7 MB disk0s1
      2: Apple_APFS Container disk2 121.1 GB disk0s2

      /dev/disk1 (internal, physical):
      0: GUID_partition_scheme *1.0 TB disk1
      1: EFI EFI 209.7 MB disk1s1
      2: Apple_APFS Container disk2 1000.0 GB disk1s2

      and I went back to use the identifier ending with disk*s2 as the name to create fusion container.
      by the way, I was using the slower Sata ssd as first argument in the ` diskutil cs create “fusion” disk1s2 disk0s2` command.

      The result turned out to be the the NVME drive speed and it seems to be very stable for the past couple hours with heavy load.

    • Xiaosong says:

      I’ve been running into the similar situation as you did, only getting SATA speed for fusion drive created using the similar command above.

      Then I recall that some other instructions, they suggest using ` diskutil cs create “fusion” disk0s2 disk1s2` instead of disk0 disk1.

      Then I went back to recreate my fusion drive, with the slower SSD coming first and NVMe SSD later.

      And I successfully get the full NVMe speed using this way.

      • Wayne says:

        Interesting. I tried with a Samsung SSD and also with a Seagate Fast SSD, both using USB-C. Both were unstable as a FUSION drive. So I am now running my Seagate as an external USB-C SSD drive.
        What enclosure is the 1TB 970 eco in? Do you have a 2018 mac mini?

  3. Wayne says:

    Yes I’ve read that the first drive needs to be the faster of the two because that’s what core storage assumes.

    My understanding is that
    disk0 is the whole hard disk
    disk0s2 is the volume

    So are you saying that using the Volume `diskutil cs create “fusion” disk0s2 disk1s2` instead of the disk disk0 disk1 might be more stable?

    Apple’s instructions specifically say to use the drive identifier. e.g. ” In the IDENTIFIER column, find the identifier for each of the two internal, physical drives that make up your Fusion Drive. Usually the identifiers are disk0 and disk1. ” See also here:

  4. Wayne says:

    Apple manual says this:

    ” create | createLVG lvgName devices …
    Create a CoreStorage logical volume group. The
    disks specified will become the (initial) set of
    physical volumes; more than one may be specified.
    You can specify partitions (which will be re-typed
    to be Apple_CoreStorage) or whole-disks (which will
    be partitioned as GPT and will contain an
    Apple_CoreStorage partition). The resulting LVG
    UUID can then be used with createVolume below. All
    existing data on the drive(s) will be lost. Owner-
    ship of the affected disk is required.”

  5. Wayne says:

    My gut reaction is that it’s nothing to do with specifying the partition or the whole drive. Maybe it’s more to do with the brand of Hard Disk. OS X was certainly having trouble with the Samsung drive, it was taking 1 minute extra to boot up whenever the Samsung SSD was connected.

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