Desktop motherboard fails to POST only when a second M.2 SATA SSD is installed in M.2_2 slot

Environment

ASUS ROG Strix Z690-G (BIOS 3801), Intel i7-12700K, 32 GB DDR4, Primary NVMe Samsung 970 EVO Plus in M.2_1, Crucial MX500 M.2 SATA in M.2_2, power supply Corsair RM750x, Windows 11 on NVMe

Symptoms

With the Crucial MX500 installed in M.2_2 the machine powers on (fans spin) but there is no POST: no video output, no keyboard LEDs, no POST beep. Removing the M.2_2 SSD restores normal POST and boot into Windows.

Observations

Removing the MX500 causes instant normal POST. The same MX500 works in a 2.5" SATA-to-USB adapter and is readable on another laptop. The M.2_2 slot appears to be physically intact, no visible standoffs touching the PCB. BIOS shows M.2_2 absent when the board is powered off and the drive is installed (no NVMe detected). Switching M.2_1 and M.2_2 (putting NVMe in M.2_2 and SATA in M.2_1) makes the system boot with the NVMe but the SATA drive then prevents POST when placed in the other slot.

Already attempted

Updated BIOS to latest stable (3801), performed CMOS reset, reseated drive and cables, tried the same SATA M.2 in the M.2_2 slot of another identical board (worked there), tried a different M.2 SATA and a second NVMe in M.2_2 (NVMe in M.2_2 boots fine), disabled SATA ports in BIOS and toggled SATA mode AHCI/RAID, powered off and cold-booted multiple times. No change when leaving BIOS defaults.

Question

Why does inserting a SATA-type M.2 SSD into the M.2_2 slot prevent POST, and what diagnostic steps and BIOS/firmware configuration changes can I try before contacting vendor RMA?

Post timeline

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AI DIAGNOSTICS

Incident analysis

Updated 1w ago• Trouble ID

Most likely causes

M.2_2 slot is wired for PCIe only and the SATA-form M.2 triggers a lane configuration the BIOS mishandles (firmware bug). 75%

Shared resources: M.2_2 shares lanes with SATA ports or a PCIe slot and the SATA device causes a bus enumeration conflict. 50%

Power-delivery issue when the SATA M.2 is inserted (unlikely given other tests), or corrupted NVM firmware on the board that mishandles SATA devices in that slot. 25%

Summary

This looks like a hardware/firmware conflict between that motherboard M.2 slot and SATA-mode M.2 devices. Many consumer boards share PCIe lanes and SATA ports; some M.2 slots only support NVMe (PCIe) while others share lanes with onboard SATA connectors and change platform configuration when a SATA M.2 is present. A SATA-type device in a slot that expects PCIe may cause the firmware to try to enumerate a device through the wrong controller, triggering a firmware bug that stalls early initialization and therefore prevents POST. Less likely but possible causes include a short/ground from the SSD to a board standoff, a defective slot, or a buggy firmware path that is only exercised by SATA-NVMe lane switching.

Suggested Fixes

Diagnostic step 1

Consult the motherboard manual to verify which M.2 slot supports SATA vs NVMe and which SATA ports are disabled when that slot is populated; confirm whether M.2_2 supports SATA at all.

Diagnostic step 2

Test the same SATA M.2 in a proven M.2 SATA-capable slot on another machine (you already did) and document the device's model/firmware; if it works elsewhere the SSD is less likely to be the issue.

Diagnostic step 3

Visually inspect the M.2 socket and standoff for metal debris or bent pins; power off and clear the slot, then reseat gently.