I love Retina iMacs. So much so that I took the chance and bought a used 2015 Core i5 iMac with a 1TB Fusion Drive (which combines a 24GB NVMe SSD and a 1TB hard drive). A machine that is actually slower than my 2012 Core i7 iMac. I bought it with an upgrade in mind, which was where the sharp finger of fate came into play.
I wanted to upgrade iMac's NVMe SSD with something that gave more space but was relatively cheaper (ie not sold by Apple). I found conflicting information about the upgrade, however. Authentically pronounced advice like "You will kill the machine and possibly yourself" was hardly useful (not to mention, totally erroneous), while "Yes, it worked in my 2017 [iMac]" hope hope, but no specific.  After purchasing the iMac (it was a bargain I had to jump on) and later talking to suppliers, it became clear that my hope of faith (and general storage knowledge) had been justified.
A disclaimer: if you bought a 21-inch iMac with a regular hard drive, you have no luck – these iMacs did not have a PCIe SSD slot on the board. But there is a profitable alternative that you can read about-NVMe via Thunderbolt. If your iMac comes with a Fusion Drive or SSD, read on.
Another disclaimer: if your iMac is still under warranty, it will void it. You also risk damaging your iMac permanently. If you are careful you shouldn't, but things happen.
This is how you can upgrade an iMac's NVMe SSD with something cheaper, faster and more spacious.
Find an NVMe SSD for the upgrade
As I thought there was never any question that the NVMe SSD could be upgraded, only if it could be done at a reasonable price. One big question is that Apple uses proprietary contacts. OWC's compatible devices at Newegg were 40 cents per gigabyte, and Apple's Amazon was an incredible $ 1 a gigabyte! This is 2019: standard M.2 2280 NVMe SSDs start at 15 cents per gigabyte!
I also found Apple compatible SSDs for about 30 cents per gigabyte. It's better, but they were from unfamiliar companies that I couldn't talk to. Finally, I stumbled upon a company with offices in Birmingham, Alabama called Fledging, whose spring drives run around 25 cents per gigabyte and are available in up to 2TB capacity. The company armed me with a number of relevant tips and sent me a 512GB M13 Feather SSD kit with MacOS High Sierra pre-installed.
While 25 cents per gigabyte is not bad, the Feather unit proved to be a good all-round executive-I was enamored with the idea of using a cheaper M.2 2280 unit. I already had a lot and they are not all cheap.
I settled on the Western Digital SN750 Black for the experiment, but you could easily do something with Crucial's P1. In addition, you need a Sinetech sub-$ 20 adapter to use an M.2 2280 SSD in an iMac.
The Sinetech adapter is actually made for MacBooks and there is a subtle difference in the distances between MacBook and iMac mount points. The end result is that the adapter is just a hair too long, and the device is just a shade more. It can still be installed, but it is not a perfect fit. Then it is again the iMac where you will never see it. Without the sight …
It has been reported that one of the most popular and fastest mid-range NVMe SSDs, Samsung 970 EVO Plus, is currently not playing well with Mac. I was going to test 970 EVO Plus as an upgrade, but while it was formatted in an external PCIe enclosure, it crashed my other Mac. I took the tip.
Installation of NVMe SSD
As I said before, this will void your iMac warranty. You also risk damaging your iMac permanently.
What is nice about Fledging NVMe is that it is a plug-and-play device. If you do not select a plug-and-play NVMe, you need to create a USB flash drive with a full copy of MacOS High Sierra or later. Third-party NVMe devices are not supported in MacOS Verions before High Sierra. You need installation-driven because you lose the ability to recover from the internet until you have re-installed macOS when replacing the main drive on a Mac.
I chose High Sierra because I know it works well with Boot Camp. Mojave's problem with Boot Camp may be over, but I chose it was right.
Another thing you might want to do is upgrade your hard drive to a 2.5-inch SSD. I did.
I'm not going to pull you off with the full cut-off instructions for a 2015 iMac. Frankly, I can't do better than the easy-to-read and granular guide on ifixit.com, which is what I used the first time. The procedure is hardly difficult, but it requires patience, a modicum of finesse and some tools.
One thing I will say is to be careful to remove the display and handle it carefully. iFixit sells a good scroll (see above) to spudge the display off which facilitates the job significantly. Use a large workspace, and remove moving something hard or point out that you might crack and break the glass. Pad below the iMac and support it so that it is tilted back.
Thereafter, the entire compartment consists of some cables, removal of some Torx screws (of course you need torx drivers), then remove the speakers, power supply (do not touch the bare solder joints on the back) and logic board. You will find the NVMe SSD on the back of the latter, secured with a screw. It's simple, but some pain points:
A power supply entry is also a screw that secures the motherboard.
There is a prison screw behind the motherboard that you have access to through a hole in the board.
The webcam and the primary displays have flip-up locks, not just on them.
Both large cables connect the power supply with the motherboard having latches. You have to press these to enable removal and the large cable requires a lot of power. Don't just cut.
There are upside-down screws behind the fan that are easy to forget.
Remember how I picked up the fit with the Sinetech adapter? I could screw the unit into place by tilting the screw with its tip just into the hole and then pushing it against the unit. Unfortunately, this caused a bit of an arc in the SSD, but should not be a problem. If you are brave, you can deepen the adapter and SSD indentations a bit with a round file and much care.
Once you have installed the unit and are ready to test it, I recommend that you only reassemble (minimum screws, leave the display untouched, no speakers etc.) so that you do not have to repeat the entire removal again if something should be failure with the device or procedure otherwise not working.
When the computer is running again, start with the USB disk you created, run the High Sierra Disk Utility to partition the drive and you should be on the move. Then complete the re-assembly process.
Which Computers Are Running
There is no PCIe SSD slot in 21.5-inch iMac devices that were not shipped with Fusion Drives or SSDs. And even though the 2012 iMacs (something with a Fusion Drive and any 27-inch model) had NVMe connectors, they were just SATA, so you can also save yourself the trouble and upgrade the hard drive to an SSD.
In 2013, the PCIe bus deployed, and although these SSDs were AHCI types, most computers are said to support NVMe. But only two or four PCIe 2.0 paths were used, so you don't have to go overboard with performance specifications. You are quite limited to 900Mbps (2 lanes) / 1.5GBps (4 lanes) throughput.
With the late 2015 iMac, PCIe revved to 3.0 so you can use even the fastest devices available, such as the WD SN750 Black that I chose.
Regarding performance. Much of the NVMe kick comes from the difficult fast (10X SATA) search times offered by each device I tested except one, which was still three times faster than SATA. The larger throughput offered by the more expensive units will only play with larger transfers. Several gigabytes transfers.
If you regularly work with large video files or write large amounts of data, you want the pricer types. Also, stay away from smaller capacity (256 GB or less), which often has fewer chips and slower writing. Avoid QLC units (Quad Level Cell / 4-bit) and cheaper TLC units where the throughput drops drastically when they run out of cache memory. Check out PCWorld's SSD reviews for individual model information.
If you shut down your hard drive during the upgrade and find your fans spinning at full speed, there is a simple action from CrystalIdea called Macs Fan Control, allowing you to fine-tune all Mac fan settings and eliminate the need for an expensive cable .
I wanted faster and I got it. Great. Directly below are the numbers for the original 24 GB SSD. Not bad, but not bleeding edge of any standard.
Next up is the simple, affordable plug-and-play upgrade Fledging Feather M13 SSD. It's also good to come with High Sierra pre-installed, but for $ 5 you can have a USB stick that lets you install Mojave.
Please note that the writing results the company sent me were quite higher than this, but they were obtained with a 2015 i7 MacBook Pro. The CPU makes a difference.
As you can see, Feather M13 is a great reader. But it varied from a good (1.2GBps) to mediocre (600Mbps) authors, depending on whether the cache was full or not. Regardless, 2.8GBps reads and the flash aims to make you still get the NVMe kick in your pants, and of course you don't have to worry about a little incredible adapter.
Only if you are not so sanguine about opening your iMac, here's the alternative I talked about-NVMe via Thunderbolt. Below are the results of the same 2015 i5 iMac, with an Akitio Thunder2 PCIe housing that accommodates a Samsung 970 EVO SSD. The older 970 EVO works well (I've run High Sierra from it for over a year), where 970 EVO Plus doesn't.
Although NVMe over Thunderbolt is not always as smooth an experience as an internal NVMe SSD (it seems as if every little break), it's still faster, but not cheap, a good option if you're shy about open your system.
Last but not least, the Sinetech adapter is connected to a 1TB WD Black SN750 NVMe. This is what I had in mind – nice and consistently large numbers across the board.
Don't listen to the doom-and-gloomers in the forum, you most definitely can upgrade the NVMe SSD in your iMac to something faster and more spacious without explaining your life's savings. I tested in 2015, but I have seen 2017 models upgraded, and Fledging assured me which Mac with SSD contact can be upgraded.
Given the price, quality and speed of plug-and-play Fledging Feather, it's probably the best option for the average user. On the other hand, I am quite satisfied with the WD Black SN750 and the adapter, despite the slightly weak fit.
Anyway, it's an upgrade that makes you smile. And not in the small part because you survived taking your Mac apart.
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