Model A1419 / EMC 2806 & EMC 2834/ Late 2014 or Early 2015 or late 2015 / 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 Processor / Retina 5K display

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Replacing HD portion of Fusion Drive with SSD


I want to replace the HD in my iMac 27" 5k systems Fusion Drive with a SSD.

Can I run this iMac with a 24 G flash drive on motherboard (apparently this flash drive is not yet upgradeable?) and this new SSD (in the place of the HD)

Essentially I would have 2 SSD drives (one big and one small)

How do I set it up after the install?

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Can you give us the make and model of the SSD you want to put in to replace the current HD. You also talk about upgrading the current SSD as well here which is a 24 GB SSD? Are you asking if you can put in a 2 TB PCIe blade drive in in place of what you have now? Also what is the exact config your iMac has now HD & SSD.



I am not sure what make HDD Apple use

This is a fairly new 27 inch Imac with a 1T fusion drive . It is not a hybrid fusion drive-it is as it came from apple.

My conern is if i just replace the HDD portion of the fusion drive with a SSD and leave the 24G flash portion as is in the motherboard wil the imac just runnir do i need to change the setup or settings of the hardware.

thx for the help


OK, lets review then what you have: a Stock 1 TB HD and a 24 GB SSD setup as a Fusion Drive. FYI: Apple slims 1 TB Fusion Drive down to a measly 24 GB of flash storage


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Frankly, I would not upgrade this system as it has a faster Thunderbolt external port than what the internal HD SATA III port offers.

Lets look at the numbers:

  • SATA III - 6.0 Gb/s
  • Thunderbolt 1 - 10 Gb/s
  • Thunderbolt 2 - 20 Gb/s

So depending on which exact model you have you might have a TB2 interface which is much faster!

Now lets look at the PCIe interface the blade SSD is currently using

  • PCIe 2 Lane (AHCI) - 10 Gb/s
  • PCIe 4 Lane (NVMe) - 32 Gb/s

So if you wanted a faster system using an internal drive I would invest the effort replacing the blade SSD with a much bigger SSD unit. Apple offers a 1 TB bade SSD but it will be expensive! If you have the older PCIe 2 Lane (AHCI) model there are some 3rd party options coming out this Spring so I would wait. So far there is no PCIe 4 Lane (NVMe) option other than Apple. In either case Apple uses a custom SSD so you can't use the now more common M.2 blade SSD's.

If you really want to replace your current HD you have some issues as you will need a special cable harness to replace the HD's thermal sensor. This is what you'll need: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later and you'll need this frame: OWC 2.5" to 3.5" Drive Adapter Bracket to hold your SSD.

Opening up the 'Thin Series' iMac's is a big under taking! Make sure you follow the IFIXIT guides to the letter as you can damage the display assembly quite easily if you don't use the correct tools and technique.

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Thx for your very informative posts and help

Just a last question- Surely the port speeds arent the limiting factor here- The solid state drive or currently the HDD read /write speeds are the limiting factor

Anyway looks like it might be best to connect an external harddrive to my firewire connectors ( both mine are 20Gb/s) - less risk and hassle

Not keen to go for the flash drive on PC board as thats almost a complete disassemble of my iMac!


Yes the read/write speed of a single drive device is less than the the throughput of the SATA III port it is connected to as well as less than the external ports max speed.

BUT! We wouldn't use a single drive device if we want to saturate the link. This is where RAID comes in. By ganging up multiple drives we can increase the I/O speed beyond the limits of a single SATA port {in this case SATA III (6.0 Gb/s)} as well as hit the external ports limit. This would be true with either a HD RAID set or a SSD RAID set.

Here's a good vid explaining the different RAID setups CompTiA Training - RAID


Ok thx

how would RAID work? Is it part of the iMAC system/ setup already or is it something that i have to setup.

Also if it is part iMAC setup can i change it or do i need to change it if i replace the HDD with say a SSD?


FYI: Your system has Thunderbolt not FireWire (800 Gb/s) if it's a 5K iMac. Make sure you get the correct external drive system.


Here's one example of an external RAID enclosure: OWC ThunderBay 4 RAID 5 Edition. We have a few of these setup with HD's as well as others with SSD's.


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There's a 1TB Western Digital Black and a 24 GB Apple SSD in your computer.

You can essentially replace both of them.

The hard drive is the usual 3.5" SATA slot. While the Apple SSD slot uses a M.2 adapter.

So something like a 850 Pro will do. I would only do this if I was a very experienced computer specialist. Disassembly the Apple Mac 5K is a very risky process and can result in potentially damaging cables or shocking yourself.

iFixit Disassembly guides here:

Hard Drive:

iMac Intel 27" Retina 5K Display Hard Drive Replacement

Solid State Drive:

iMac Intel 27" Retina 5K Display Blade SSD Replacement

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If i replaced both the DD for a 1TB SSD and replaced the 24gb PCIe SSD with a 128gb PCIe SSD what would be the process of configuration? Would I need to create a RAID system or will the fusion drive automatically be created when I re-install OSX?


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Hey Dan, how do you know what PCIe Lane model you have? I have the 5K Late 2015 27" iMac 3.2 i5 with the 24GB Fusion Drive. I bought this about 10 months ago and thought I was getting 128GB Otherwise, I would got the 2TB one. I think I'm willing to pay for it within reason, but how do I purchase one? Thanks. I been reading your posts and the info you have is very informative.

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Here is your systems specs: iMac 27" 3.2 GHz i5 - 5K (Late 2015)

This system has a custom Apple PCIe 2.0 x4 NVMexpress interface. Apple sold this system with either custom a PCIe x2 SSD (when intro'ed) or a PCIe x4 SSD (currently sold systems). Sadly, getting a larger SSD for your system is going to be expensive and hard to find as it's a custom part made for Apple. To see what your system has you'll need to look under the System Report section when you open 'About This Mac'

Lastly, this system is a bear to open and you'll need to pull the logic board to get to the blade SSD interface. Here's the IFIXIT guide: iMac Intel 27" Retina 5K Display SSD Replacement

Again, I recommend going with an external RAID drive (SSD being the fastest). Your only other option is to buy a new system that meets your needs and sell yours. But when you do the math I think you'll see the external is the better direction from a cost and risk perspective.


I have the PCIe x4 NVMe. My local authorized repair shop can get me a 256 GB for $500-599 (they were not sure on my exact price and I have a buddy who works there).

Yikes! I now have a RAID1 NAS with large HDD's and it's slow, over the network. I use it to store all my raw files and video projects.

So I could move everything over to a Thunderbolt SSD RAID. But, I'd still like to have more SSD storage in this Mac.

I understand the SATA III is slower than the PCIe. But, would this make loading programs and files faster than it currently is?

I usually work from the local internal drive and then move the finished project to the NAS when I am done. I am now leaning towards this and my buddy can install it for me for free. That would be more cost effective than replacing the blade SSD at this time.

So if I replace the internal HDD for a SSD it won't be Fusion drive as both drives are then SSD's. Do you use them in parallel or the blade SSD as a scratch drive? Thanks your for input.

And yes perhaps I should buy a new machine...


Nome -

RAID-1 is just disk mirroring which just means it offers a safety net if one of the drives dies. This is not faster than what a single drive's I/O would be. To add to that an Ethernet connection is not very fast when compared to Thunderbolt. It makes a lot of sense for holding your projects that need to be shared. I would recommend moving to RAID-5 on your NAS if you have space for the extra drives if you do a lot of sharing between systems.

What you need is a RAID-0 drive set if you want speed. We have three 4 drive RAID-0 drive sets (both HD & SSD units) The HD RAID-0 is much faster than RAID-1 and our SSD RAID-0 just smokes!

It sounds like we're doing about the same thing here. Yes, swapping out your HD for a SSD will improve performance as the HD is not able to fill the SATA III (6.0 Gb/s) channel unlike the 2.5" SATA III SSD can so its better from that perspective. You'll need to follow the IFIXIT guide here: iMac Intel 27" Retina 5K Display Hard Drive Replacement and you'll need this from OWC: In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later as you're taking away the HD's thermal sensor the system requires. And you'll need a 2.5 to 3.5 frame: OWC 2.5" to 3.5" Drive Adapter Bracket.


You do need to break the Fusion Drive set before you take the HD out as well as make a good backup.

As to setting up the two SSD's afterwards: You'll need to leave them as discrete drives as you can't run them as a RAID as the I/O channels (& drives) are not the same speed. You'll just bog down the PCIe drive as it can't go any faster than the SATA drive (if you are using software RAID Vs hardware which won't work at all). A Fusion Drive also won't work here as the way it was designed was only when used with a HD/SSD combo. Here the OS can't discriminate between the two SSD's so it can't set the faster drive as the biased drive.

I would setup the PCIe SSD as the OS & Apps drive. Don't put anything else on it as you want the free space for the OS's virtual RAM, Apps cache & paging files. Then run the other for any loose files and your work files.


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hello guys I'm at the exact same problem .

Just one question: Can I remove the SSD blade and the SATA HDD and connect a 1 TB SSD there? Is it possible?

Many thanks in advance

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This system has two drive ports one SATA (HDD) and one PCIe (SSD). When you buy a Fusion Drive model you are using both of these ports, If you buy a HDD only model then you have an empty PCIe port. If you buy a SSD model you have an empty SATA port.

OK, so you really want an internal upgrade. You'll want to first break the Fusion Drive set (if you have that) before you pull either drive.

Now the question what are you replacing the HDD or the SSD? While tons harder I would replace the PCIe SSD with a bigger unit (1 TB is possible! But very costly and very fast). The easier path is to replace the HDD with a SSD. But its still a big project and not very fast Vs the PCIe port. Doing either you could damage the display in the process. An expensive Opps!

OK, what would I do with such a new system?

I wouldn't open it! I would instead get a Thunderbolt external SSD RAID drive as its cheaper and easier than replacing the built-in SSD and much faster than swapping out the SATA HDD with a SSD.


Hello Dan and many thanks for your answer.

Since I have a 1Tb SSD I'm willing to try and replace the SATA HDD by the SDD.

My question is: is it possible? Have been reading a lot and can't reach a conclusion...


There is a difference between 'Willing' & 'Wise' ;-}

I don't think it's this is wise move given the amount of work and the risks here.

Yes, you can swap out the SATA HDD for SSD drive. You'll need a few parts to do it besides the SSD. You'll need a frame to hold the smaller SSD drive: OWC 2.5" to 3.5" Drive Adapter Bracket and you'll need this thermal sensor to replace the one thats in the HD for the system to work correctly: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later and you'll need the tool and strips in this kit: iMac Intel 27" (2012-2015) Adhesive Strips

Again, I would recommend you go with an external Thunderbolt2 SSD drive as being a faster and cheaper option (given the risks). Don't for get the SSD here will be slower than what the Thunderbolt connected drive can offer.


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