SN850 1TB NVME - slow write speeds (3100)

Hi all,

First time post here, so hopefully someone can help.

I have an SN850 1TB setup as my OS drive in M2_1. All onboard devices like audio and wifi are disabled in BIOS.

I’m only getting about 3100 write speeds, fluctuating very slightly but obviously far lower than expected.

I’m running a 5950x, with an MSI MEG X570 Godlike on BIOS 1.D1. Latest AMD chipset drivers. The 850 itself is on the very latest firmware, due to go to the app later this week I believe but downloadable now.

I’ve read that M2_1 being chipset could mean the lanes are being throttled perhaps and the write speed is definitely, consistently about half or so of what it should be.

Prior to this going in, I had a Samsung 980 Pro 500GB that was working just fine until about a month ago after a firmware update, where the write speeds dropped to about 1200, about 25% of expected and nothing from that side of support was worth the effort.

Additionally, I have another 850 1TB in M2_3 and that’s operating just fine, with expected writes.

Does anyone have any ideas? On the surface, it feels M2 slot and driver/software/firmware in nature and that sort of bears out with the M2_3 slot operating correctly.


1 Like


I have exact same issue with slow write speeds (3300MB/s) when SN850 runs on chipset lanes.

I have 5950x and Asrock X570 ITX/TB3 all updated with latest firmware and drivers.

So far tried the 1TB and 2TB SN850 drives and both have this exact same behaviour.

Ticket open with Asrock who confirmed the issue in their lab of slow write speeds on chipset lanes.

Asrock opened a ticket with WD who replied that this is a known issue when running this drive on chipset lanes.

I asked Asrock to explain further on the cause but they refused to explain telling me that they are under NDA from WD.

Full thread here: WD SN850 Slower than expected performance | Page 5 | TechPowerUp Forums

I have the same issue, SN850 2TB, 3200MB/s in chipset slot, 5200MB/s in CPU slot. Considering to return this drive to amazon and get a different brand.

SN850 2TB
ASUS ROG Crosshair VIII Hero WiFi
RTX 3090

Hilarious reply from WD Support…

“If the device is working fine in all slots except the M2.1 slot, then unfortunately it is most likely a firmware issue with this specific motherboard model. We cannot provide any solution as this is the motherboards manufacturers issue. We apologize for the inconvenience.”

Off to MSI I go, see what they have to say but if ASRock are aware of a problem with these drives on the chipset lanes, and are under NDA to not disclose further, the problem may be fundamental and unfixable.

We’ll see.

I’ve just stumbled upon this reddit post that says:

"Boards like the X570 Hero Crosshair VIII have the 2nd M2 slot connected to the chipset. Although this slot is supposedly X4 Gen4 Pcie it bottlenecks new, fast SSDs like the 980 Pro and sn850. In the primary M2 slot you may get 7000/5000 MB sec read/write but in the 2nd M2 you may get 6500/3200, way below the x4 Gen4 pcie spec.

Asus support confirmed in my support case that there may be significant latency induced when using the 2nd M2 due to the chipset and this causes lower speed. I was surprised to hear this."


That doesn’t match my findings.

On the MSI MEG x570 Godlike, M2_1 and M2_2 are PCH, M2_3 is CPU.

But only M2_1 is throttling, so the first of the PCH slots. The others, which includes PCH and CPU, are working at full speed, M2_3 using an 850, M2_2 using a 980 Pro 1TB.

Also, before I swapped to the 850 for my OS drive on M2_1, I had a Samsung 980 Pro 500GB and prior to Samsung screwing up the turbo write and binning out write speeds, it was also working at full speed (and the Samsung firmware due end April fixes this turbo write clearout problem too).