2019 iMac Fusion Drive Failure — Upgrade HDD to SSD or also Remove Failed NVMe?

lot of respect going on here:

Australopithicus01 >> breaking and cleaning off the adhesive is tedious, but...I find slicing the adhesive tape using a scalpel with the blade guided by the aluminium edge gives clean and easy cut

i highly recommend using a plastic CUTTING WHEEL this is the one that comes in the strip adhesive kits i buy off AMZ:

this tool separates (cuts) the factory adhesive like butter in under 5 minutes -- the removal jobs you want to beware of are the ones that have been previously opened and resealed -- there is great risk if not experienced in removing screen assemblies (ive had them take over an hour...tediously working through the cement)

Australopithicus01 >> the SN770 is a great performer in this iMac

i will risk repeating myself for anyone installing a Western Digital NVMe SSD in a 2019 (slot under the main board) -- until you get more specific opinions -- if they work on 2019 and Sequoia -- because my experience is they have issues that make them undesirable -- not a sleep issue! its a "3rd party NVMe controller. Loss of MMIO space. Read. fBuiltIn=1 MODEL=WD_BLACK SN850X HS 1000GB FW=620361WD CSTS=0xffffffff US[1]=0x0 US[0]=0x7 VID=0x15b7 DID=0x5030 CRITICAL_WARNING=0x0.\n"

Here is the crash report consistent with five 2019 imacs upgraded with WD_BLACK SN850X SSDs -- more than 20 of these same reports on the five Macs -- i uninstalled all of them and went exclusively to SSPOLARIS OEM Apple SSDs -- no issues since

panic(cpu 0 caller 0xffffff8012bf4c9a): nvme: "3rd party NVMe controller. Loss of MMIO space. Read. fBuiltIn=1 MODEL=WD_BLACK SN850X HS 1000GB FW=620361WD CSTS=0xffffffff US[1]=0x0 US[0]=0x7 VID=0x15b7 DID=0x5030 CRITICAL_WARNING=0x0.\n" @IONVMeController.cpp:5713
Panicked task 0xffffff9046e7b980: 252 threads: pid 0: kernel_task
Backtrace (CPU 0), panicked thread: 0xffffff8b7a570b30, Frame : Return Address
0xffffffe9b994fa20 : 0xffffff80109d8721 
0xffffffe9b994fa70 : 0xffffff8010b4de96 
0xffffffe9b994fab0 : 0xffffff8010b3cfec 
0xffffffe9b994fb70 : 0xffffff801096d971 
0xffffffe9b994fb90 : 0xffffff80109d8a17 
0xffffffe9b994fc90 : 0xffffff80109d809b 
0xffffffe9b994fd00 : 0xffffff80111d146d 
0xffffffe9b994fdf0 : 0xffffff8012bf4c9a 
0xffffffe9b994fe20 : 0xffffff80111015d3 
0xffffffe9b994fe70 : 0xffffff80111014f6 
0xffffffe9b994fea0 : 0xffffff8010a37a78 
0xffffffe9b994fee0 : 0xffffff8010a38ac8 
0xffffffe9b994ffa0 : 0xffffff801096d19e 
      Kernel Extensions in backtrace:
         com.apple.iokit.IONVMeFamily(2.1)[C90BCF94-6412-3ADB-BA60-8F49E3213E29]@0xffffff8012bef000->0xffffff8012c1afff
            dependency: com.apple.driver.AppleMobileFileIntegrity(1.0.5)[0609D106-50A6-3AD1-A00A-3A58CB25E6A2]@0xffffff80120d5000->0xffffff801210ffff
            dependency: com.apple.iokit.IOPCIFamily(2.9)[18240D5E-B3B6-3F5A-B567-486D9CAA7C15]@0xffffff8012e85000->0xffffff8012eb9fff
            dependency: com.apple.iokit.IOReportFamily(47)[20EEB3ED-6CA5-30ED-B214-C69C5EAFF776]@0xffffff8012eca000->0xffffff8012eccfff
            dependency: com.apple.iokit.IOStorageFamily(2.1)[0F68373A-6845-334F-A3F9-1547BE96F804]@0xffffff8012fc3000->0xffffff8012fd9fff


Process name corresponding to current thread (0xffffff8b7a570b30): kernel_task

Australopithicus01

I should note:

I installed Sequoia on my five internal sn850x SSDs and ran them as my boot system

they were new sn850x 1TB, 2TB, 4TB heatsink models

macOS Sequoia was installed fresh on each 2019 iMac from a thumbdrive macOS installer, updated, with only a few apps installed -- DriveDX, AJA System Test

this is the second stage of my testing process on Macs I buy to flip

these were the first sn850x SSDs I installed under Sequoia so I kept them running for weeks with sleep set to normal (I did not prevent the units sleep) to test them over time -- im glad I did

first, I erase and install fresh Sequoia, update, test around on the original fusion volume to test the unit for any anomalies

second, I replace the fusion Apple SSD with sn850x SSD (and remove the SATA HDD)

then erase the sn850x and install fresh Sequoia, update, test around before sealing the iMac back up

the sn850x-Sequoia units tested solid all around -- but started hitting the "3rd party NVMe controller. Loss of MMIO space" restarts after I updated from 15.3 to 15.4x -- I kept the update process going through 15.6x hoping the issue would be resolved -- but it didn't

that's when I replaced them all with the genuine authentic OEM SSPOLARIS parts and all issues cleared

I add this to establish the state of the installed systems -- this testing occurred mid-late 2025

my whole point of all this is save someone the headache of installing Western Digital sn850x SSD because they are a pain to swap out a second time... if this information flies here

akfromnyc wrote:

Here is the DriveDx report - I have never used this before, just downloaded it as you suggested:

https://discussions.apple.com/content/attachment/b6032251-5ab1-43d7-a90f-64293302a701

Please post the complete DriveDx text for each of the internal drives. I would like to examine the full report for that SSD. I have sometimes been able to resurrect SSDs which report errors. Errors are not always fatal with an SSD.

The health report for the internal Hard Drive also shows a problem. Attribute #189 "High Fly Writes" has exceeded the manufacturer's expectations. This is a cause of concern. I don't think I've ever seen this occur on any of the Hard Drive's I've examined.

It seems the other health attributes for the internal SSD look good. I'll have to think about the best way to have you reset the internal SSD to see if it can improve the SSD's performance & condition. It has been a while since I performed the operation....much harder to provide instructions. However, I'm not sure it really matters since your internal Hard Drive is of concern so recreating the Fusion Drive isn't a good idea. While the internal SSD is 120GB.....installing macOS onto it won't give you any room to store data on it. And until we know the SSD's errors are not continuing to accumulate, using it for data storage is not recommended.

Installing macOS onto the internal SSD may be an interesting exercise and monitoring the health of the SSD to see if those "Data & Media Integrity Errors" continue to increase & at what rate, especially if we reset the internal SSD.

I think the combination of these problems likely broke the Fusion Drive.

I don't think either of the internal drives will cause a problem at this time for booting macOS from an external SSD.

akfromnyc wrote:

Separately, I find the DriveDX usage figures a bit puzzling. The SSD shows ~15,000 hours (and the HDD ~25,000) over ~5.5 years. But given that I’m away roughly 3 months a year and, when home, realistically have ~6 hours/day of potential usage time, the implied “active” window is closer to ~9,000 hours total. The reported SSD figure exceeds that by a wide margin, which suggests it’s capturing powered/idle/background time rather than true user activity—but at face value, the math doesn’t intuitively line up with how much I’m actually on the machine. It may also reflect background processes like Backblaze, Time Machine, and Dropbox keeping the drives intermittently active even when I’m not actually using the Mac?

The difference in power on hours between the SSD & HD is probably due to the SSD being able to enter a low power mode whereas the HD is not. It is much easier for an SSD to enter a low power mode than it is for a HD.

And as long as a Mac is powered on, the drives will also be powered on unless the system is configured to power down drives. macOS tends to kill HDs by being overly aggressive at putting HDs to sleep since macOS tends to wake up a lot during "sleep" periods.....and that sleep & wake is very hard on the HD's drive heads. I've seen a lot of Apple HDs wear out at less than 5k power on hours due to this aggressive sleep/wake of HDs.

So yes, the HD at least is still awake & considered "powered on" even when you are not actively using the device. Some of it may be due to configuration, but also to the items you have mentioned causing the Mac to wake up to perform maintenance functions & scheduled tasks. I've never taken the time to analyze the power on hours of a Fusion Drive setup.

Australopithicus01 >> WD Black SN770

Hi, what macOS are you running that SN770 in the 2019 NVMe slot

i presume with Sintech NGFF adapter

I had WD_BLACK SN850X SSDs running flawlessly in five 2017-2019 iMacs under Mojave and Sequoia but started getting overnight crashes around 15.4 and replaced them all with OEM SSPOLARIS SSDs — Mojave still ran them perfect

now i only install the OEM SSPOLARIS SSDs…the Samsung 970 EVO Plus SSDs were perfect in Mojave and Sequoia but they got too expensive

I'm sure it is easy for some users. I have myself taken apart many Macs and no issue. However on my 2019 27" iMac I find using an external TB NVME drive to be a much easier solution.

Thank you for this information -g.

Cutting Wheel

Your example seems like the same iFixit product I tried. I was not happy and I turned to the unthinkable steel blade because I have the skills. Happily we both know the cable issues and we each do what we find works best. Possibly your product is sharper than the iFixit which would make it a safer suggestion for most.

Loss of MMIO space

Your documented information is most helpful and persuasive. It also reinforces this article citing issue on older MacBooks. https://theaveragelearner.com/post/how-to-fix-3rd-party-nvme-controller-loss-of-mmio-space-macbook-kernel-panic/

My reference for WD Black came from information by trs96 in tonymac86 and my own limited experience.

I was considering replacing the SN770 with the SN850x as internal boot drive but your information gives pause for thought. By design the two drives are very different, the SN850x with onboard NAND and the SN770 with HMB.

While I have not read any unsatisfactory reports of 850x in 2019 imac in MacRumors, your experience becomes most important.

I originally purchased the 4TB 850x as an external drive which works well in an external Zike Z666 and thus good for a future M4 Mac mini upgrade. With the SN770 behaving well there is little incentive to replace. So there it shall remain.

Many Thanks.

Are you reporting success with SN770 internal on iMac 2019 running SEQUOIA?

the sn850x were solid internal running MOJAVE on 2019 where i do my heavy lifting

the “crash” running Sequoia is you get a ‘your computer was restarted’ notice when you wake it up in the morning

then the ‘report to Apple’ prompt

then the detailed report

sometimes good for a week, then the morning crash report

sometimes two days in a row it would happen

i had sn850x SSDs installed in five 2019 iMacs over a few months

all five 2019 iMac running sequoia with internal sn850x had the issue

2019 are my thing

I’ve upgraded at least twenty 2019 fusion models with NVMe SSDs in the past two years

i flip them for a good time…

Hi -g,

I test new releases of OSX on an external drive Zike drive with WD Black SN850x NVMe, to get the feel of them. I have also run both Sonoma and Sequoia on the iMac with SN770.

Given your comments, I cannot say I have run either of those two operating system on my iMac long enough to make an informed statement. I can say during the short periods they were installed, about one week, there was no evidence of any technical incompatibilities. No boot problems whatsoever.

The SN770 has operated smoothly, once I had removed the Sintech 2280 and fitted a cheap short NFHK adapter. I have principally been using Ventura so your point about Sequoia is moot. You have made me curious and I will be interested to test Sequoia for a longer period.

Reflecting on the SN850x, I recollect many of the successful upgrades on MacRumors relate to the earlier SN850.

Another question that deserves review. Thanks again for your information.

Hi tbirdvet,

My apologies for contradicting your statement about ease of iMac upgrade. I interpreted your comment as relative in comparison with other iMacs. I now realise you meant in relation to an inexperienced user. A comment with which I can fully agree.

Thank for your reply which under the circumstances is most generous:

I'm sure it is easy for some users. I have myself taken apart many Macs and no issue. However on my 2019 27" iMac I find using an external TB NVME drive to be a much easier solution.

Yes. I agree. Saying "easy" grossly overstates the process from the perspective of the uninitiated. I withdraw that remark. I was thinking in terms of other iMacs whereas I should have been thinking in terms of the challenge to the novice. Thank you for your corrective advice.