MSI B450M MORTAR MAX: PCI_E4 stops working after M.2

Identify the affected connectors

Read the printed labels, not just the physical position. PCI_E4 is the lower full-length expansion slot; M2_2 is the second storage socket. The primary graphics slot and first M.2 socket have different connections.

The MAX suffix matters. Use its specification and manual rather than a diagram for another MORTAR version.

Test the sharing explanation

1. Record which SSD occupies each M.2 socket and which card occupies PCI_E4.
2. Compare the failure with the installation date. A card that was already failing needs a broader diagnosis.
3. If practical, power down, disconnect mains and temporarily remove the M2_2 device following the manual.
4. Check whether the card is detected again. If that SSD held the operating system, account for the missing boot drive when evaluating the test.

Choose connections that can coexist

M2_1 accepts PCIe or SATA M.2 storage; M2_2 uses PCIe 2.0 x4. Moving an SSD depends on its interface, the first socket's availability and CPU-specific support. Physical fit alone is not enough.

If M2_1 is occupied, decide whether the second SSD or the card in PCI_E4 is more important. A different supported storage connection may solve the allocation problem. Before buying an adapter, check the bandwidth and clearance of the remaining slots.

Test one change at a time. Keep the original cable and socket positions in your notes so you can restore a known-working arrangement.

When to investigate the card itself

Software cannot provide a second independent connection where the manufacturer specifies sharing. If PCI_E4 remains undetected with M2_2 empty, investigate the card's own requirements, seating and support.

Distinguish firmware or device detection from an application failing to use an already detected card. Those observations lead to different next steps and are more useful to support than repeatedly reinstalling unrelated drivers.