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How can I switch from FusionDrive to SSD-PCIe only?

Now I've been trying to find this answer for a while and did a bit of research online but nothing seems to match what I have in mind.

Basically I want to get rid of the slow 1TB HDD in my 5k iMac and add a Samsung 450PRO SSD.

I have the late 2015 fusion drive model and I know this one has also a PCIe SSD of 24gb that works with the 1TB HHD together.

Is there any chance of replacing this PCIe SSD with a bigger one, say 500GB and also add a Samsung 450PRO SDD so they can work together?

If I do so, how it will appear after reboot? Will it give me the option of choosing whether to install the OS on the PCIe SSD or Samsung SSD ?

I'm just worried if I make this upgrade I won't be able to see the PCIe in the system as at the moment the 24Gb one that came with doesn't show up , I can only see my 1TB HDD in Finder.

Hope this makes sense.


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1 Answer

Chosen Solution

Ah! This is a big one!

Lets start off talking about the PCIe blade SSD Apple uses. So far no one offers a 3rd party replacement SSD for this series. Frankly, I think it's going to be awhile, I would keep an eye on OWC here: iMac SSD Upgrades. With that said you might be able to find an Apple authorized service shop to order you the 1 TB spare drive but it will be very expensive! So far Apple hasn't allowed anyone to get them as spares only as a warranty swap out. But, I think they will be softening this policy soon.

Next, lets talk about why you don't see the discreet drives in the finder in a fusion drive set. This is to be expected! The OS see's the two drives as one storage space (virtual volume). To see the drives you'll need to look under the system report under 'About this Mac' There you'll see the two drives.

As to the Samsung SSD I would recommend you stick with the EVO series Vs the PRO. The cost factor is not worth it unless you are editing or creating full length movies. Here's the spec sheets for both:

FYI: There is no 450 Pro SSD option here.

OK, we've tackled the easy questions so far so what to do?

Yes, you can break the Fusion drive set so each drive (HDD & SSD) are seen discreetly Vs as a virtual drive you have now. Sadly, you can't create a fusion drive set using two SSD's. So swapping out the HDD for a SSD while doable won't give you the max performance you could get here. As the replacement SSD will be limited to the HD's SATA port speed: SATA III (6.0 Gb/s). I also must warn you opening this system is a bear! And if you don't use care you can damage the display assembly which would be a very expensive mistake!

Instead of altering your internal storage how about going a different direction, I would recommend going with an external Thunderbolt2 RAID drive set (RAID 0) using SSD's. Here we can achieve faster performance than the built-in SATA III port effectively driving 2 or 4 drives in parallel! 2 drives 6.0 Gb/s = 12 Gb/s theoretical or 4 drives 24 Gb/s theoretical.

Once you set it up you can then switch the startup (boot drive) to be the external drive, gaining the its speed. Sadly the internal drives are of little use at that point.

Here is what we use: OWC ThunderBay 4 mini.

Update (03/03/2017)

OK so you're going for the internal SSD solution:

Here's the deal you first need to break the Fusion Drive set. Follow the instructions here: Mac World - How to split up a Fusion Drive

Next you'll need some tools and the tape strips to replace the ones that currently hold your display on. Follow this IFIXIT guide to letter! If you don't Opps! expensive replacement display: iMac Intel 27" Retina 5K Display Display Replacement and here's the guide to replace the strips at the very end of the job: iMac Intel 27" Retina 5K Display Adhesive Strip Replacement

OK, now the fun begins!

Follow this IFIXIT guide (jumper to Step23) to take your current HD out: iMac Intel 27" Retina 5K Display Hard Drive Replacement. Here we are swapping out the 3.5" HD for the 2.5" SSD which use the same SATA interface. We are not changing the custom PCIe blade drive.

You'll need your SSD at this point and you'll need this as well: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later and you'll need an adapter frame as the older drive is 3.5" and the SSD is 2.5". As you have to order the thermal sensor why don't you get this as well from OWC: OWC 2.5" to 3.5" Drive Adapter Bracket

You NEED the thermal sensor no and's If's or But's!

The System Management Computer (SMC) within your system uses sensors to manage its cooling. Once you disconnect one, the system goes into failsafe driving the fans into high speed overtime you'll burn out the fan's besides who wants to hear them.

No, don't use fan control software it really doesn't work long term. All you'll do is end up either killing your fan or your system. Trust me I've tried and I'll never do it again!

iMac Intel 27" Retina 5K Display Adhesive Strips Image


iMac Intel 27" Retina 5K Display Adhesive Strip Replacement



iMac Intel 27" Retina 5K Display Display Image


iMac Intel 27" Retina 5K Display Display Replacement



30 - 45 minutes

iMac Intel 27" Retina 5K Display Hard Drive Image


iMac Intel 27" Retina 5K Display Hard Drive Replacement



1 - 2 hours

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Spot on, this is very helpful and a lot on info that I didn't know where to find.

Thanks so much for your effort and time to answer my question!

I think for now I will just swap the HDD with a Samsung 450 PRO, I have tried both before in a laptop and 450 is definitely faster, and yes I am editing movies and doing animation and 3D so I need the fastest SDD I can get.

Following tho this decision there's another thing that got me confused when it comes to swapping the 1TB HDD with an SSD.

I noticed some people add the new SSD drive into a frame they buy from online/shop along with a TEMPERATURE sensor but some people don't and even mention you don't need a TEMP sensor. Very confusing.

They also just take out the HDD and install the new SSD with exactly the same cable but some use that SATA adaptor.

I'm not sure the late 2015 iMac needs that SATA adaptor or not or even a TEMP sensor .

Would much appreciate if you know any answers to this as well.

Best wishes,



Remember the Samsung 950 Pro drive will NOT work in this system (if that is what you meant)! The PCI'e blade drive Apple uses is not the same.

Click on the links I posted these are Samsung's current crop of 2.5" SSD's that will work in your system. There is one other the Samsung 750 EVO thats it.

I've added the needed steps and parts above. I still think the external direction is the better here.


You are just a small spot off, OWC Aura is a fitting 3rd party replacement SSD for this series.

edit add: OWC does not market it to iMac audience, though it should work. Parts SSDAB2MB02, SSDAB2MB05, SSDAB2MB10.


@unsubstantiated - What? Thats not a compatible PCIe blade drive They just offer a 2.5" SSD which is the same as a Samsung 2.5" SSD. See for your self here: iMac 27" Late 2013 — Current

Sorry still no joy here: The rub is the PCIe lane width! The OWC PCIe is only a two lane part (they are using a RAID 0 AHCI SSD setup which is not quite as fast as a native PCI SSD) the newer systems are 4 lane PCIe.

While it would offer more storage you loose on I/O speed (sllooowww). Besides, its even more work to get to and I'm sure no one has even given it a try. I can do better externally using a Thunderbolt2 RAID drive set.


Sorry Dan, I ment to say 850PRO, don't know why I wrote 450 or what was in my mind lol. Gotcha anyway, I knew already how to open the iMac etc, just was very confused about that Thermal Sensor but will definitely follow your steps now.

Thanks so much for your help mate.

Have a good weekend!


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