MSI X870 Tomahawk M.2 slots: USB4 and PCIe sharing explained
· By Weerasak
Document-based guide · Methodology and limits
Quick answer
For most builds, M2_1 is the simplest first-SSD slot and M2_4 is the least complicated second slot. M2_2 shares an upstream link with the rear 40Gbps USB-C controller, while M2_3 interacts with the lower PCI_E3 slot. The exact result also changes with the installed Ryzen family, so choose a layout from the board table rather than from an M.2 slot number alone.
The short answer: M2_1 first, then choose around USB4 and PCI_E3
If you have one NVMe SSD, M2_1 is the cleanest default on the MSI MAG X870 TOMAHAWK WIFI. MSI connects it to the processor and does not attach either of the board's two published sharing footnotes to that socket. For a second drive, M2_4 is the uncomplicated choice because MSI documents it as a chipset-connected PCIe 4.0 x4 socket without the M2_2 USB4 condition or the M2_3 PCI_E3 condition.
That recommendation is a layout decision, not a promise that one socket makes every SSD faster in real applications. The installed processor, SSD controller, workload, thermals and firmware can all affect measured performance. The purpose of this guide is narrower: preserve the connections you actually need and avoid discovering a disabled or reduced-width device after the PC is assembled.
Exact four-slot map
| Socket | Source and documented maximum | Important condition |
|---|---|---|
| M2_1 | CPU; PCIe 5.0 x4 with Ryzen 9000/7000, PCIe 4.0 x4 with Ryzen 8000 | No M.2 sharing condition is attached to M2_1 in MSI's specification. |
| M2_2 | CPU; PCIe 5.0 x4 with Ryzen 9000/7000, PCIe 4.0 x4 with Ryzen 8700/8600/8400 | Unavailable with Ryzen 8500/8300. With supported CPUs it shares bandwidth with the rear 40Gbps USB-C ports. |
| M2_3 | X870 chipset; PCIe 4.0 x2 | Installing a drive makes PCI_E3 run at x2. Selecting PCI_E3 x4 in BIOS disables M2_3. |
| M2_4 | X870 chipset; PCIe 4.0 x4 | MSI does not attach an M.2 lane-sharing footnote to this socket. |
The table is a transcription of the MSI specification for this exact motherboard. Do not transfer its labels to another board: M2_2 on one model can have an entirely different route from M2_2 on another. Also note that “no attached sharing footnote” is limited to MSI's published table; it is not a claim that every device combination has been independently tested.
What M2_2 does to the rear 40Gbps USB-C connection
MSI says the rear USB 40Gbps Type-C ports and M2_2 share PCIe 5.0 x4 bandwidth. When a device is installed in M2_2, MSI documents both sides as running at PCIe 5.0 x2. The board also provides a BIOS choice that gives M2_2 PCIe 5.0 x4, but selecting it disables the rear 40Gbps USB-C ports. MSI separately says the USB4 host controller itself supports up to PCIe 4.0 x4.
This does not mean that inserting an SSD automatically disconnects USB4. Under the documented shared setting, the M2_2 route and controller remain present at the stated x2 widths. The disabling event occurs when M2_2 is explicitly switched to x4 in BIOS. Likewise, the x2 notation describes the controller's PCIe-side route; it is not a measured file-copy rate for a dock, display, enclosure or capture device.
If a high-bandwidth USB4 workflow matters, use M2_1 and M2_4 before M2_2. If a second CPU-connected Gen5 SSD matters more and the USB4 ports are not needed, the documented M2_2 x4 BIOS option may fit the plan. If both matter, retain the shared setting and evaluate actual workload measurements after the build rather than assuming that theoretical link width predicts application time.
What M2_3 does to PCI_E3
M2_3 and the lower PCI_E3 expansion slot form a different tradeoff. MSI lists M2_3 as PCIe 4.0 x2. When M2_3 contains a device, PCI_E3 also runs at x2. The BIOS can switch PCI_E3 to x4, but doing so disables M2_3.
A full-length connector does not guarantee sixteen electrical lanes. PCI_E3 is the lower slot MSI documents at up to x4, and this sharing rule does not describe the primary PCI_E1 graphics slot. A capture card, network adapter, storage controller or other add-in card may have its own minimum or preferred link width. Check that card's specification before deciding that x2 is sufficient.
At the same PCIe generation, x2 has half as many lanes as x4. PCI-SIG explains the relationship between generation, lane count and aggregate bandwidth. That ratio is a transport limit, not a universal 50 percent application-performance result. If the PCI_E3 card requires x4, select PCI_E3 x4 and leave M2_3 unavailable. If the card works at x2, the documented shared arrangement can retain both devices.
The processor changes the storage map
| Installed processor group | M2_1 | M2_2 | Extra note |
|---|---|---|---|
| Ryzen 9000 or 7000 | Up to PCIe 5.0 x4 | Up to PCIe 5.0 x4, subject to USB4 sharing | The M2_3 and M2_4 chipset routes remain as listed above. |
| Ryzen 8700, 8600 or 8400 group | Up to PCIe 4.0 x4 | Up to PCIe 4.0 x4, subject to USB4 sharing | Use MSI's exact grouping rather than treating every Ryzen 8000 processor alike. |
| Ryzen 8500 or 8300 group | Up to PCIe 4.0 x4 under MSI's Ryzen 8000 entry | Unavailable | MSI also limits the rear USB 40Gbps controller route to PCIe 4.0 x2 for this group. |
AMD identifies the Ryzen 7 9800X3D as a Ryzen 9000-series AM5 processor; AMD identifies the Ryzen 5 8600G and Ryzen 5 8500G as separate Ryzen 8000-series products. Those AMD pages identify the CPUs, but the motherboard-specific socket behavior comes from MSI. Do not infer M2_2 availability from the shared “Ryzen 8000” family name.
Choose a layout by the devices you need
| Build goal | Straightforward layout | Known tradeoff |
|---|---|---|
| One boot SSD | M2_1 | None of the two documented M.2 sharing conditions is invoked. |
| Two SSDs while preserving USB4 and PCI_E3 | M2_1 + M2_4 | The second drive is on the chipset at up to PCIe 4.0 x4. |
| Two CPU-connected SSDs on Ryzen 9000/7000 | M2_1 + M2_2 | M2_2 invokes the documented USB4 shared mode unless it is switched to x4 and USB4 is disabled. |
| Three SSDs with an x4 card in PCI_E3 | M2_1 + M2_2 + M2_4 | Avoid M2_3; M2_2 still has the USB4 decision. |
| Three SSDs while keeping M2_2 empty | M2_1 + M2_3 + M2_4 | PCI_E3 is limited to x2 if it is also occupied. |
| All four onboard M.2 sockets | M2_1 + M2_2 + M2_3 + M2_4 | Accept the M2_2/USB4 shared state and the M2_3/PCI_E3 x2 state; CPU restrictions still apply. |
“Straightforward” means the arrangement follows the published route table with fewer conflicts. It is not an endorsement of a particular SSD, RAID level or thermal result. Four adjacent drives can also introduce cooling and power considerations that a lane diagram does not answer.
A safe check before moving a drive
- Record the exact motherboard name and CPU. Do not substitute the MAG X870E TOMAHAWK WIFI or another revision.
- Photograph the printed socket labels. Use M2_1 through M2_4 from the board and manual, not “top,” “middle” or “the slot under the GPU.”
- List every device that needs USB4 or PCI_E3. Include docks, external storage, displays and add-in cards before choosing the SSD sockets.
- Check the current manual and BIOS notes. Use the MSI support page because menu names and firmware behavior can change. Do not copy a BIOS path from a different MSI board.
- Back up important data and disconnect AC before hardware changes. Follow the motherboard, case and SSD installation instructions; do not move a module while the system is energized.
- Verify each device after assembly. Confirm that firmware enumerates every SSD and that the required USB4 and PCIe devices remain present. Treat negotiated width and workload performance as separate checks.
Common X870 Tomahawk M.2 questions
Does M2_2 disable USB4?
Not in the default shared arrangement described by MSI. With a supported CPU and a device in M2_2, MSI says M2_2 and the USB 40Gbps controller both operate through x2 links. Choosing M2_2 x4 in BIOS disables the rear 40Gbps USB-C ports.
Which socket should hold the fastest SSD?
M2_1 is the clean first choice because it is CPU-connected, supports the board's highest documented generation for Ryzen 9000/7000 and does not invoke the USB4 or PCI_E3 sharing notes. Real performance still depends on the SSD and workload.
Can a Ryzen 5 8500G use M2_2?
No under MSI's published 8500/8300 condition. M2_2 is marked unavailable. Use the Ryzen 5 8500G lane guide to compare how other exact boards route additional SSDs.
Does M2_3 slow the graphics card?
The cited M2_3 sharing rule concerns PCI_E3, the lower slot, not PCI_E1. CPU-specific limits for the primary graphics slot are a separate part of the specification and should not be inferred from this M2_3 rule.
Do any M.2 sockets disable SATA ports on this board?
MSI's exact specification lists the M2_2 conflict with USB4 and the M2_3 conflict with PCI_E3; it does not attach a SATA-disable note to these M.2 sockets. Do not generalize that result to another motherboard.
Should I force every available link to x4?
No. The x4 choices intentionally remove another route: M2_2 x4 disables USB4, while PCI_E3 x4 disables M2_3. Choose according to the connected devices, then measure the workload if bandwidth matters.
Sources and review scope
Specifications checked October 22, 2026. Slot routes, widths and sharing outcomes come from MSI documentation for the MAG X870 TOMAHAWK WIFI. AMD pages identify the named processors, while PCI-SIG supplies link-width context. This document-based guide does not benchmark an SSD, USB4 device, graphics card or add-in card, and it does not extend the findings to the X870E model or another Tomahawk revision.