During a recent discussion on Official Apple Support Community, ’-g‘ reported a test
with five x 2019 iMacs upgraded with a WD Black SN850x each of which resulted in crashes.
Shortened form of generated error report:
panic(cpu <span>0</span> caller <span>0xffffff8012bf4c9a</span>): <span>nvme:</span> <span>"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"</span> @IONVMeController.<span>cpp:</span><span>5713</span><br>Panicked task
Given the prevailing opinion that WD Black SN850x is compatible with iMac 2019, it seems that compatibility may end with OSX 15.3.
The object of this post is to gather evidence of actual SN850x performance and incidence of MMIO space loss or other performance errors.
However online discussions of "Loss of MMIO space" reveal causes other than NVMe brand. See:
theaveragelearner.com
Would MacRumors upgraders with WD Black SN850x or SN770 NVME please
provide feedback regarding good or bad performance - particularly on
OSX Sequoia 15+
This will help us all understand the limitations of SN850x SN770 for 2019 iMac.
"Macs tested:
One 2019 5k i5 with matched set OWC RAM (4x32gb)*
One 2019 4k i7 with 64gb matched set OWC RAM (2x32gb)*
One 2019 4k i5 with 16gb (2x8gb) Apple RAM
Two 2019 4k i7 with original 32gb Apple RAM (2x16gb)
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 thumb drive macOS installer, updated, with only a few apps installed -- DriveDX, AJA System Test.
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. 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".
with five x 2019 iMacs upgraded with a WD Black SN850x each of which resulted in crashes.
Shortened form of generated error report:
panic(cpu <span>0</span> caller <span>0xffffff8012bf4c9a</span>): <span>nvme:</span> <span>"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"</span> @IONVMeController.<span>cpp:</span><span>5713</span><br>Panicked task
Given the prevailing opinion that WD Black SN850x is compatible with iMac 2019, it seems that compatibility may end with OSX 15.3.
The object of this post is to gather evidence of actual SN850x performance and incidence of MMIO space loss or other performance errors.
However online discussions of "Loss of MMIO space" reveal causes other than NVMe brand. See:
How To Fix "3rd party NVMe controller. Loss of MMIO space" MacBook Kernel Panic - The Average Learner
A common problem for people who has their MacBook SSD upgraded - How I've found a weird way to fixed mine.
theaveragelearner.com
Would MacRumors upgraders with WD Black SN850x or SN770 NVME please
provide feedback regarding good or bad performance - particularly on
OSX Sequoia 15+
This will help us all understand the limitations of SN850x SN770 for 2019 iMac.
"Macs tested:
One 2019 5k i5 with matched set OWC RAM (4x32gb)*
One 2019 4k i7 with 64gb matched set OWC RAM (2x32gb)*
One 2019 4k i5 with 16gb (2x8gb) Apple RAM
Two 2019 4k i7 with original 32gb Apple RAM (2x16gb)
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 thumb drive macOS installer, updated, with only a few apps installed -- DriveDX, AJA System Test.
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. 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".
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