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!

3 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.

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