Replacing HD portion of Fusion Drive with SSD

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?

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.

Dan '

Sorry for butting in but how do you 'break" the fusion drive?

I have a 2019 21" iMac with 1tb fusion drive and want to upgrade both the PCIe SSD and the 1tb HD. I thought it would be a simple process but found out like you mentioned the need to "Break" the fusion drive...so for now I've given up, I've searched some on the web and could only find how to repair a fusion drive.

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.

Hi. I have late 2014 27inch iMac. My hdd just died. Should I replace it or just go external and keep applications and system on the internal 128 ssd? Thanks.

I've never had any problems. Both my iMac 27" & Mac Pro have Thunderbolt connected drives.

didnt work on mine. I replace the OEM 1tb mechanical drive with an SSD. Blackmagic speeds were the same or slower than before.

Sadly, A dual SSD Fusion Drive is not really better than a HDD & SSD setup. Its best to break the fusion drive set and use the SATA SSD alone, Or if you can upgrade the PCIe SSD to 512 GB or better. Then use it as your boot drive with your apps, and use the SATA drive as your data drive.

Hi @danj - None of the iFixit tutorials talk about breaking the fusion drive before replacing an HDD with an SDD. So I have to think many (most) folks following the iFixit tutorial are not doing that. Is the fusion drive perhaps breaking just by virtue of a new SSD drive being installed? Man I thought I was ready to get to work on this tomorrow, but not so sure now.