Help solve mystery of: iMac Fusion drive SSD slot

[AndreeOnline]

Guys:

I bought a used 27 iMac late 2015 with an 4GHz i7.

As it turns out it has been configured with a Fusion drive, but not your vanilla kind. It has a 24GB Intel SSD (I guess that's stock) for the 'fast' SSD part and then it has a 1TB Samsung 850 EVO for the 'large' part. All well and good, kind of.

But I want to open it up and break the Fusion drive and put a larger SSD where the small Intel sits and use that as a separate drive from the Samsung EVO. I get that breaking the Fusion config is wiping the slate clean and that is what I want. Saving data is of no concern.

On OWC's site they offer complete Aura Pro X2 SSD kits for iMac late 2013 to current. But strangely, their compatibility guide really says:

Note my computer bottom left. So there are both older and newer computers compatible (with a note that some requires they were shipped with a Fusion drive), but for some reason mine isn't.

OWC support woman claimed "the chart is correct, but I don't know why". I don't hold 'not knowing' against her, I just want to be sure that it actually is correct, and why this exact model won't work.

I don't want to open the iMac up unnecessarily.

Any insight is appreciated, as always!

 

[AndreeOnline]

Hmm... it seems that, as irony would have it, this exact model—when the buyer opted for a Fusion drive—Apple did some wonky solution where the SSD part doesn't sit in a normal PCIe connector on the back of the board like on all other iMacs....

..but if you bought this computer with a normal HDD you actually DO have that slot!! 🤯😂

Still hoping that someone here opened their iMac up and can provide more fact and new options!

 

[AndreeOnline]

Nah, I still think there's hope.

Excellent article on Apple proprietary SSDs here:

Beetstech SSD article

 

[TwoH]

This honestly baffles my mind why it wouldn't be compatible with Late 2015s...
But for 2015 late and mid I upgrade the SSD - I simply install with an original SSUBX flash.

 

[nambuccaheadsau]

Have a read of this from ifixit and their recommendation to replace the bladr drive with a 256GB version.

SOLVED: Can you use two SSD’s as a Fusion Drive set? - iMac 27" 2017

We did some tests a few years ago and found the smaller 32/42 GB Blade SSD’s just didn’t offer enough benefit in a Fusion set when mated with a SATA SSD. It was more beneficial as the boot drive and only holding the MacOS files and nothing more. Apps & data files would be held on the SATA...

www.ifixit.com

 

[AndreeOnline]

I've done some more research in the meantime. The 17.1 iMac should have the required NVMe slot in Apple's format: 12+16 pins.

I spoke to OWC technical support (not sales this time) and he asked to get back to me later via email. He just did and said it won't work on my model. But the answer was really much more of "we don't know why it's listed as incompatible and no one is comfortable with saying it now is".

I am now pretty confident that it would at least be a physical fit for the connector. But if OWC says that about their drive, then what about other ones? There is so much FUD out there it's hard to weed out the good information.

It's annoying since while I don't mind tearing the iMac down, I would get slightly frustrated doing that for nothing.
[automerge]1570203795[/automerge]

Have a read of this from ifixit and their recommendation to replace the bladr drive with a 256GB version.

Thanks. That links to the site I mention higher up: Beetstech. They seem knowledgeable, but they also list that exact drive as incompatible with my 17.1.

 

[TwoH]

All I can suggest on the matter - make sure you test it properly before you stick the screen back down if you use that drive, and I'd also try doing a deep sleep with it and waking it up to see if it kernel panics or not, as it could follow the same issues that happen with NVMe and SSPOLARIS flash ssds in 2015 iMacs

 

[AndreeOnline]

Is it perhaps that my iMac uses drives based on the AHCI protocol, and not NVMe, even though they share the same physical connector?

But that would mean that anything older than my iMac also wouldn't work, even if it's listed 'incorrectly' above.

 

[tsialex]

Is it perhaps that my iMac uses drives based on the AHCI protocol, and not NVMe, even though they share the same physical connector?

But that would mean that anything older than my iMac also wouldn't work, even if it's listed 'incorrectly' above.

No, all supported Macs got the NVMe EFI module support with High Sierra firmware upgrades. Apple did it even to Macs that don't accept a PCIe blade internally, but have Thunderbolt ports, like Mac mini 2011/2012 and classic 2011/2012 13"/15" MBPs.

iMac17,1 NVMe EFI module in the 173.0.0.0.0 BootROM, right below to the AHCI support:

Apple ROM Version
  BIOS ID:      IM171.88Z.F000.B00.1909131902
  Model:        IM171
  EFI Version:  173.0.0.0.0
  Built by:     _atsserver@xapp157
  Date:         Fri Sep 13 19:02:14 2019
  Revision:     173 (B&I)
  ROM Version:  F000_B00
  Build Type:   Official Build, RELEASE
  Compiler:     Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
  UUID:         F0617C92-AC78-3D2F-8DA8-041C791BB575
  UUID:         1FB6DAA2-C815-34AC-BABE-68D68CB50952

 

[zedex]

[REPOSTED AND MERGED FROM MAC PRO FORUM]

I thought I'd read something about this issue in my endless education on the Mac implementation of PCIe SSDs....

You were really close (see this section on the Beetstech website): https://beetstech.com/blog/apple-proprietary-ssd-ultimate-guide-to-specs-and-upgrades

EMPHASIS - this is about the specific weirdness of your model - read WHOLE THING

BEETSTECH offers an exceptional explanation of the anomolies (by model) needed to navigate this space for success..

[you can save 20% by sourcing it from eBay rather than BT]


POST 2

I know why other well computing are saying that it is completely unsupported solid state drive. Don't make the mistake of purchasing that AURA X.

You need a specific type of NVMe SSD that can be formatted in 4Kn

The good news is that they are cheaper and faster than the SSD that you are presently contemplating purchasing. MORE TO COME..

 

[AndreeOnline]

Thanks. My drive, the 24GB part, is indeed listed as a NVMe unit.

I also found the other thread on the iMac sub forum discussing various SSD projects. It doesn't exactly build confidence as to what exactly is needed for success as it's a bit of hit and miss.
[automerge]1570213987[/automerge]

EMPHASIS - this is about the specific wierdness of your model - read WHOLE THING

Thanks. Yes, I had read that earlier today and thought that held some promise and makes it sound like it's compatible with models of today—such as the OWC drive.

But you mention other drives being a better choice. Do you have a specific drive in mind? 1TB would be the sweet spot I think. I'm not looking to sink too much money into this iMac project.

 

[zedex]

OK - here we go!

Comprehensive research, differential diagnosis and deductive reasoning have all led me to conclude the following...... Apple engineers decided to experiment with the 2015 27 inch iMac by installing a Samsung – not Intel – 32GB [24GB usable] NVMe PCIe SSD.

This is the earliest implementation of the NVMe protocol on record as every AHCI-based PCIe SSD installed by Apple in every other Macintosh model - from 2015 all the way back to 2012 - categorically required an AHCI-based PCIe SSD in order to comply with the older [original] firmware/EFI system boot framework that first allowed Macs to boot from a SATA compatible AHCI-based PCIe SSD when Mac OS 10.10 YOSEMITE was first released.

So here's the rub..

Most cMP users knew the 140 firmware update gave us all the ability to natively boot from a NVMe PCIe SSD beginning with the release of High Sierra.. Only High Sierra wasn't around when your 2015 model 27-inch iMac was released to the market with its experimental 32GB NVMe PCIe SSD.

Through diligent research and a lot of help from members of the Mac Pro forum – a very under-appreciated discovery was made. Under specific conditions (4Kn formatted NVMe disks) you could actually boot from an NVMe PCIe SSD in macOS 10.12 SIERRA.. so how could this be..?

It stands to reason that your 2015 iMac 32GB NVMe PCIe SSD must have been formatted at 4096 bytes per sector (4Kn) in order to host macOS 10.12 (Sierra).

Ergo, all Mac users can use an NVMe PCIe SSD from SIERRA upwards on the condition it is correctly formatted (and all thanks to your special iMac release).

Were it not for this isolated iMac NVMe experiment, the NVMe firmware update (FW140.00 or equivalent) distributed to all Macs would only have supported the '512 bytes per sector' ultimately adopted by Apple across the spectrum in Generation 5 PCIe SSDs pursuant to the default disk format / LBA (512b) of the SSPOLARIS NVME-based controller.

In closing - the SSUBX (or SM951) disks are compatible with everything (expensive though) but a Sabrent 1TB (properly formatted) or a Toshiba XG5 2TB (properly formatted) is faster, bigger and cheaper.

The level of complexity here means a company like OWC will never market or stock a NVMe PCIe SSD substitute/solution to this particular model of iMac because for every five units sold, 4 would be returned.

PS - verify that your model was actually shipped with the requisite PCIe connector before undertaking the arduous task of tearing down the iMac for installation.

Further – you'll need to buy a small adapter off EBAY (VERY CHEAP) to augment the pinning if you ultimately decide to go with one of the third-party SSDs I've mentioned above.

BOOM - SOLVED (that was an epic walk-through!)

 

[tsialex]

OK - here we go!

Comprehensive research, differential diagnosis and deductive reasoning have all led me to conclude the following...... Apple engineers decided to experiment with the 2015 27 inch iMac by installing a Samsung – not Intel – 32GB [24GB usable] NVMe PCIe SSD.

This is the earliest implementation of the NVMe protocol on record as every AHCI-based PCIe SSD installed by Apple in every other Macintosh model - from 2015 all the way back to 2012 - categorically required an AHCI-based PCIe SSD in order to comply with the older [original] firmware/EFI system boot framework that first allowed Macs to boot from a SATA compatible AHCI-based PCIe SSD when Mac OS 10.10 YOSEMITE was first released.

So here's the rub..

Most cMP users knew the 140 firmware update gave us all the ability to natively boot from a NVMe PCIe SSD beginning with the release of High Sierra.. Only High Sierra wasn't around when your 2015 model 27-inch iMac was released to the market with its experimental 32GB NVMe PCIe SSD.

Through diligent research and a lot of help from members of the Mac Pro forum – a very under-appreciated discovery was made. Under specific conditions (4Kn formatted NVMe disks) you could actually boot from an NVMe PCIe SSD in macOS 10.12 SIERRA.. so how could this be..?

Therefore, that iMac 32GB NVMe PCIe SSD must have been formatted at 4096 bytes per sector (4Kn) in order to host macOS 10.12 (Sierra).

Ergo, all Mac users can use an NVMe PCIe SSD from SIERRA upwards on the condition it is correctly formatted (and all thanks to your special iMac release).

Were it not for this isolated iMac NVMe experiment, the NVMe firmware update (FW140.00 or equivalent) distributed to all Macs would only have supported the '512 bytes per sector' ultimately adopted by Apple across the spectrum in Generation 5 PCIe SSDs pursuant to the default disk format / LBA (512b) of the SSPOLARIS NVME-based controller.

In closing - the SSUBX (or SM951) disks are compatible with everything (expensive though) but a Sabrent 1TB (properly formatted) or a Toshiba XG5 2TB (properly formatted) is faster, bigger and cheaper.

The level of complexity here means a company like OWC will never market or stock a NVMe PCIe SSD substitute/solution to this particular model of iMac because for every five units sold, 4 would be returned.

PS - verify that your model was actually shipped with the requisite PCIe connector before undertaking the arduous task of tearing down the iMac for installation.

Further – you'll need to buy a small adapter off EBAY (VERY CHEAP) to augment the pinning if you ultimately decide to go with one of the third-party SSDs I've mentioned above.

BOOM - SOLVED (that was an epic walk-through!)

No, this is incorrect.

Today's NVMe EFI Support, of all Macs, are totally agnostic about bytes per sector and it's only a question related to macOS NVMe driver support for people wanting to run later builds of El Capitan and all builds of Sierra. Since Apple initially only used NVMe drives that had 4096 bytes/sector with some of the 2015+ Macs and only added software/driver support for these 4096 bytes/sector, later builds of El Capitan and Sierra can only boot from 4096 bytes/sector NVMe drives. High Sierra on wards, Apple started macOS support for NVMe drives with both sector sizes. 512 bytes/sector is commonplace with the consumer market but 4096 bytes/sector is very common with enterprise and datacenter markets and is not an Apple eccentricity whatsoever.

An iMac17,1 with current BootROM and running High Sierra/Mojave/Catalina/recent builds of Windows 10 supports NVMe drives with 512 or 4096 bytes/sector.

 

[tsialex]

Btw, the easiest way to check bootable NVMe support for a specific Mac is to use a Thunderbolt to M.2 adapter. If it boots from the M.2 installed in the Thunderbolt adapter, you can install it internally with 12+16 connector using an adapter to standard M.2, like the Sintech commonly used with MP6,1 and 2013-2015 MacBooks Pro.

 

[AndreeOnline]

This sounds good...

...leading me to believe I have some options.

What bugs me is going into the thread here where many SSD transplants have been made, many on updated computers, and it turns out you can't wave your arms without hitting a guy/gal with hibernation/wake problems.

Silver lining: the reason I'm not quite up to date on the SSDs is that I bought the SSUBX for my Mac Pro early on... long before PCIe really took off (and NVMe was unheard of) and it's been a 'set it and forget it' deal. Still going strong. But that means I have that drive as a fallback. So if I buy something that won't work in the iMac, maybe I can just transfer that to the Mac Pro and put the SSUBX in the iMac. That actually seems like the easiest—or at least most likely to succeed—solution.

 

[tsialex]

This sounds good...

...leading me to believe I have some options.

What bugs me is going into the thread here where many SSD transplants have been made, many on updated computers, and it turns out you can't wave your arms without hitting a guy/gal with hibernation/wake problems.

Silver lining: the reason I'm not quite up to date on the SSDs is that I bought the SSUMX for my Mac Pro early on... long before PCIe really took off (and NVMe was unheard of) and it's been a 'set it and forget it' deal. Still going strong. But that means I have that drive as a fallback. So if I buy something that won't work in the iMac, maybe I can just transfer that to the Mac Pro and put the SSUBX in the iMac. That actually seems like the easiest—or at least most likely to succeed—solution.

Yes, you can use any type of NVMe U.2/M.2 drives with a MP5,1 and you won't have any firmware upgrade problems related to the non-OEM drive. With some of the newer Macs, like MP6,1, you can't upgrade BootROM with a non-Apple SSD installed internally.

 

[zedex]

In fairness - I always defer to Alex - but at least on this occasion we happen to be saying very similar things. Though once again - I feel obliged to tip my hat to him, because without that post I would never have known that bootable NVMe PCIe SSD started - not from SIERRA - but from El Cap. Cheers for that!!

Also - I've owned a few SSUBX - there is a lot to be said for that 'set and forget' benefit.

Btw, the easiest way to check bootable NVMe support for a specific Mac is to use a Thunderbolt to M.2 adapter. If it boots from the M.2 installed in the Thunderbolt adapter, you can install it internally with 12+16 connector using an adapter to standard M.2, like the Sintech commonly used with MP6,1 and 2013-2015 MacBooks Pro.

This is both a safe and effective means of testing and you need something like this to change the drives listed in the link below to 4Kn.

...and you don't need 4Kn if you never want to go back to 10.11 or 10.12

PCIe SSDs - NVMe & AHCI

A big thanks to @atonaldenim for itemising the "high compatibility" purchasing options in the NVMe SSD space. I purchased the Sabrent Rocket (1TB) about 6 weeks ago and it is EXCEPTIONAL. This model is pre-formatted as 4Kn or 4096 bytes per sector. In my own opinion this makes the SABRENT ROCKET...

forums.macrumors.com

 

[studioj20]

Well I thought this upgrade was working but I’m getting kernal panic restarts every 20 min or so. I’ll look a little into the sector formatting issue a bit. Luckily I hadn’t sealed up the system yet!

 

[studioj20]

I unchecked “Turn hard disks off when possible” in energy saving settings and the kernel panics -seemed- to have stopped. But will keep testing. I checked the sectors for this drive using some terminal commands I found and both device and format showed at 4096. I think another post here was suggesting that maybe this Aura m2 was 512...

 

[studioj20]

Welp it’s still restarting after kernel panic just not quite as often. Here is some of the panic detail below in case anyone is interested. Someone here suggested if I found an original apple 2tb NVMe maybe it would work ok? I see some on eBay with pin adapters that look like they may work? Thanks for any info.

****

Anonymous UUID: 1737AA5D-DDFD-A1A9-77D3-877ACE29E673



Sun Feb 2 21:30:41 2020



*** Panic Report ***

panic(cpu 0 caller 0xffffff7f878b6b30): nvme: "Fatal error occurred. CSTS=0xffffffff US[1]=0x0 US[0]=0x5 VID/DID=0xffffffff

. FW Revision=S0121C\n"@/BuildRoot/Library/Caches/com.apple.xbs/Sources/IONVMeFamily/IONVMeFamily-387.270.1/IONVMeController.cpp:5334

Backtrace (CPU 0), Frame : Return Address

0xffffffa75f5fba70 : 0xffffff8003dae6ed

0xffffffa75f5fbac0 : 0xffffff8003eea185

0xffffffa75f5fbb00 : 0xffffff8003edb8ba

0xffffffa75f5fbb70 : 0xffffff8003d5bb40

0xffffffa75f5fbb90 : 0xffffff8003dae107

0xffffffa75f5fbcb0 : 0xffffff8003dadf53

0xffffffa75f5fbd20 : 0xffffff7f878b6b30

0xffffffa75f5fbe80 : 0xffffff7f878b69fa

0xffffffa75f5fbec0 : 0xffffff8003ded885

0xffffffa75f5fbf40 : 0xffffff8003ded425

0xffffffa75f5fbfa0 : 0xffffff8003d5b0ce

Kernel Extensions in backtrace:

com.apple.iokit.IONVMeFamily(2.1)[A67B3600-6FE3-3037-AC41-8C2D353B6250]@0xffffff7f878a9000->0xffffff7f878e8fff

dependency: com.apple.driver.AppleMobileFileIntegrity(1.0.5)[1D716047-7F62-3FFA-8C01-26C166B3739A]@0xffffff7f84b8d000

dependency: com.apple.iokit.IOPCIFamily(2.9)[BFDEB8D4-50FE-3DDB-87B7-F6A504393830]@0xffffff7f84695000

dependency: com.apple.driver.AppleEFINVRAM(2.1)[B83F29B8-24AC-303B-BBBA-CF332168FDE6]@0xffffff7f84d63000

dependency: com.apple.iokit.IOStorageFamily(2.1)[71BB22B0-3075-35A1-B04E-FBAC574DA80D]@0xffffff7f84983000

dependency: com.apple.iokit.IOReportFamily(47)[0BC9F93B-456A-3D97-BE4C-69DCBB5E8A3C]@0xffffff7f851f8000



BSD process name corresponding to current thread: kernel_task



Mac OS version:

18G103



Kernel version:

Darwin Kernel Version 18.7.0: Tue Aug 20 16:57:14 PDT 2019; root:xnu-4903.271.2~2/RELEASE_X86_64

Kernel UUID: C41337A1-0EC3-3896-A954-A1F85E849D53

Kernel slide: 0x0000000003a00000

Kernel text base: 0xffffff8003c00000

__HIB text base: 0xffffff8003b00000

System model name: iMac17,1 (Mac-65CE76090165799A)



System uptime in nanoseconds: 5797290500337

last loaded kext at 994794363569: com.apple.driver.AppleXsanScheme 3 (addr 0xffffff7f88506000, size 32768)

last unloaded kext at 1568942919875: com.apple.driver.AppleXsanScheme 3 (addr 0xffffff7f88506000, size 32768)

 

[tsialex]

Welp it’s still restarting after kernel panic just not quite as often. Here is some of the panic detail below in case anyone is interested. Someone here suggested if I found an original apple 2tb NVMe maybe it would work ok? I see some on eBay with pin adapters that look like they may work? Thanks for any info.

****

Anonymous UUID: 1737AA5D-DDFD-A1A9-77D3-877ACE29E673



Sun Feb 2 21:30:41 2020



*** Panic Report ***

panic(cpu 0 caller 0xffffff7f878b6b30): nvme: "Fatal error occurred. CSTS=0xffffffff US[1]=0x0 US[0]=0x5 VID/DID=0xffffffff

. FW Revision=S0121C\n"@/BuildRoot/Library/Caches/com.apple.xbs/Sources/IONVMeFamily/IONVMeFamily-387.270.1/IONVMeController.cpp:5334

Backtrace (CPU 0), Frame : Return Address

0xffffffa75f5fba70 : 0xffffff8003dae6ed

0xffffffa75f5fbac0 : 0xffffff8003eea185

0xffffffa75f5fbb00 : 0xffffff8003edb8ba

0xffffffa75f5fbb70 : 0xffffff8003d5bb40

0xffffffa75f5fbb90 : 0xffffff8003dae107

0xffffffa75f5fbcb0 : 0xffffff8003dadf53

0xffffffa75f5fbd20 : 0xffffff7f878b6b30

0xffffffa75f5fbe80 : 0xffffff7f878b69fa

0xffffffa75f5fbec0 : 0xffffff8003ded885

0xffffffa75f5fbf40 : 0xffffff8003ded425

0xffffffa75f5fbfa0 : 0xffffff8003d5b0ce

Kernel Extensions in backtrace:

com.apple.iokit.IONVMeFamily(2.1)[A67B3600-6FE3-3037-AC41-8C2D353B6250]@0xffffff7f878a9000->0xffffff7f878e8fff

dependency: com.apple.driver.AppleMobileFileIntegrity(1.0.5)[1D716047-7F62-3FFA-8C01-26C166B3739A]@0xffffff7f84b8d000

dependency: com.apple.iokit.IOPCIFamily(2.9)[BFDEB8D4-50FE-3DDB-87B7-F6A504393830]@0xffffff7f84695000

dependency: com.apple.driver.AppleEFINVRAM(2.1)[B83F29B8-24AC-303B-BBBA-CF332168FDE6]@0xffffff7f84d63000

dependency: com.apple.iokit.IOStorageFamily(2.1)[71BB22B0-3075-35A1-B04E-FBAC574DA80D]@0xffffff7f84983000

dependency: com.apple.iokit.IOReportFamily(47)[0BC9F93B-456A-3D97-BE4C-69DCBB5E8A3C]@0xffffff7f851f8000



BSD process name corresponding to current thread: kernel_task



Mac OS version:

18G103



Kernel version:

Darwin Kernel Version 18.7.0: Tue Aug 20 16:57:14 PDT 2019; root:xnu-4903.271.2~2/RELEASE_X86_64

Kernel UUID: C41337A1-0EC3-3896-A954-A1F85E849D53

Kernel slide: 0x0000000003a00000

Kernel text base: 0xffffff8003c00000

__HIB text base: 0xffffff8003b00000

System model name: iMac17,1 (Mac-65CE76090165799A)



System uptime in nanoseconds: 5797290500337

last loaded kext at 994794363569: com.apple.driver.AppleXsanScheme 3 (addr 0xffffff7f88506000, size 32768)

last unloaded kext at 1568942919875: com.apple.driver.AppleXsanScheme 3 (addr 0xffffff7f88506000, size 32768)

You are having NVMe related KPs, do you have the newest firmwares for your iMac and M.2 blade? For the BootROM you can check my table for 10.15.3 here, for your M.2 you have to check with the manufacturer.

Apple 12+16 OEM blades will always work better than M.2 blades and adapters, but you can make it work if you have the current firmware, the recommended adaptor and a compatible M.2 blade.

 

[studioj20]

Thanks! It seems this particular iMac though never received an updated bootrom for standard nvme, see

A list of successful iMac 27" (2012-2019) SSD upgrades

Here you can find and discuss all the collected knowledge regarding upgrading the HDD, blade SSD and CPU of a 27" iMac 2012-2019, as well as a list of performed upgrades. Here is a list of all 27" iMac models subjected in this thread: Late 2012 - 13.2 - MD095LL/A (2.9Ghz i5-3470S, HDD only /...

forums.macrumors.com

I’ve tried the listed kernel command to see if this stops the panics. if not I’ll try to seek out an original 1tb Apple blade perhaps?

 

[tsialex]

Thanks! It seems this particular iMac though never received an updated bootrom for standard nvme, see

A list of successful iMac 27" (2012-2019) SSD upgrades

Here you can find and discuss all the collected knowledge regarding upgrading the HDD, blade SSD and CPU of a 27" iMac 2012-2019, as well as a list of performed upgrades. Here is a list of all 27" iMac models subjected in this thread: Late 2012 - 13.2 - MD095LL/A (2.9Ghz i5-3470S, HDD only /...

forums.macrumors.com

I’ve tried the listed kernel command to see if this stops the panics. if not I’ll try to seek out an original 1tb Apple blade perhaps?

All Macs with Thunderbolt ports got BootROM bootable NVMe support, even 2011 MacBook Pros and Mac minis.

All Macs from 2011 and even some from 2010 like 2010 Mac Pro have full NVMe support. The question is if the NVMe EFI firmware module that Apple added to the BootROM have support for NVMe power management, Apple didn't intended that people used M.2 NVMe blades internally with most Macs and didn't make the module capable to manage power states for non OEM drives. So it's a problem that you have to overcome, since some blades will have excessive power usage or are just incompatible.

Apple 12+16 OEM blades like SSUAX and SSUBX are fully supported with your iMac.

 

[studioj20]

Ah thanks. Is there a 2tb oem blade out there that I can buy? if so I don’t suppose you have a link with the right one? Thanks so much for the help.

 

[tsialex]

Ah thanks. Is there a 2tb oem blade out there that I can buy? if so I don’t suppose you have a link with the right one? Thanks so much for the help.

Apple had a SSUAX 1TB drive version specific for 2013 Mac Pros, it was a BTO option at the time, but it's a very wide drive that I think that only fit to 2013 Mac Pros if used with the original heatsink, people remove the heatsink to use with MacBook Pros. You have to research this to see if will fit your iMac, looking at Mac Tracker the biggest SSD for your iMac was 1TB too. Btw, it's a extremely expensive drive.

Apple only had 2TB drives with 2017 iMac, those are SSPOLARIS and are NVMe.