Replacing HD portion of Fusion Drive with SSD

"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/sThunderbolt 1 - 10 Gb/sThunderbolt 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/sPCIe 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."

"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"

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.

"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"

"I have been researching an answer for a few days. The definitive answer seems a little allusive. I am A+ cert. and also have graduated Google training. I don't have any issues performing the upgrade. My question is ""Will a 1 T SSD installed in place of the original 1 T HDD that is part of the original Fusion drive work? "" As we know, Apple is a little funny about what plays nice with their hardware/software. The logic board has a 32 G Samsung SSProton blade 12x16 pin PCIe 3.0 x 4 interface. Pretty sure this is Apple proprietary. This is the main reason for this question/comment - Will it work if I replace the Seagate 1T HDD (model ST 1000DM003) with a Samsung 1T 860 Pro SSD? I have the sata/thermal cable adapter and frame for 3.5 to 2.5 drive as well as new adhesive strips. Any knowledge I will appreciate.

Thanks Dan for the response. I was afraid that would be the solution. Only problem is the expense of the 3.0 x 4 SSDs. 256 is $400. Gotta love Apple."

Although the suggestion of using an external thunderbolt connected SSD is a good idea, I would like to point out that on the iMac, and possibly all thunderbolt connectors are a poor design. The cable/connector interface is too fragile and loose in my opinion. My external thunderbolt connected drives are constantly disconnecting whenever I tilt the display or move cables near the drive ends. And yes, I have tried multiple thunderbolt cables. So if you want a reliable boot drive, go with an internal SSD.