Why isn’t my second 4TB M.2 SSD detected?

That board should support multiple NVMe drives, so lane limits are unlikely with your Ryzen 5 9600X and a normal setup. Load optimized defaults or update the motherboard BIOS, then check whether the drive appears under the NVMe or storage information page. If it remains completely absent, test the 4TB drive in another computer or use an NVMe-to-USB enclosure. That will separate a motherboard-slot issue from a faulty SSD.

The Fanxiang drive could simply be defective, especially if it is not detected in BIOS after reseating. A new, uninitialized SSD would still normally show up in BIOS and Disk Management, so formatting is not the first thing to troubleshoot. If it fails in multiple M.2 slots or another system, return or replace it.

Since the drive is missing from the BIOS, this probably isn’t a Windows partition or formatting problem. Try the 4TB SSD in each M.2 slot, checking that it is fully inserted and secured with the correct standoff. Also remove any protective film from the thermal pad and make sure the heatsink isn’t preventing the drive from sitting flat. If it works in another slot, check the motherboard manual for slot-specific sharing or configuration rules.

If the existing 2TB drive is in the first slot, try placing the new drive in the second M.2 slot and temporarily disconnecting SATA devices or other PCIe expansion cards. Some boards share chipset lanes with certain ports, although that usually disables a slot rather than making the drive disappear everywhere. Also confirm the drive is an NVMe model and not a SATA M.2 drive, since the two interfaces are not interchangeable.