One module shows up in the BIOS and the other does not. Both are the same size, you bought them together, and both fit the slot. That points to one of three things: one module is faulty, the two modules are not alike in a way the board cares about, or the slots are not equal. A short test sorts them. This guide gives the test, the table of differences that matter, and the fix for each. See also POST troubleshooting hub for the full no-start checklist.
The short answer
- Test each module alone in the same slot, then in a second slot. MemTest86 recommends removing modules to isolate a failing one and notes that slots are not equal.
- If one module fails alone in every slot, it is faulty or the wrong kind for the system.
- If both work alone but fail together, a mixing or population rule is the cause.
- Compare the part numbers. A pair that looks alike can differ in type, ECC, density or rank.
The test
- Power off, unplug and remove both modules.
- Install module A alone in the slot the manual lists first. Boot and check the BIOS memory page.
- Repeat with module B in the same slot.
- Swap slots. Test each module in the second slot. Moving a module also re-seats its contacts, which can clear a fault.
- Install both in the layout the manual gives for two modules.
Reading the results
| Result of the one-at-a-time test | What it points to |
|---|---|
| Module A works in slot 1 and slot 2; module B fails in both | Module B: faulty, wrong type, or a density or capacity the system does not support |
| Both modules work alone in the same slot; they fail together | A population or mixing rule; MemTest86 says this often points to a multi-channel configuration problem |
| A module works in slot 1 but not slot 2 | The slot, or a rule about which slot is filled first |
| Both modules fail in slot 2 but work in slot 1 | The slot |
| Both fail everywhere | A system-level cause: BIOS, board, processor or profile |
Differences that make a board accept one module and not another
| Difference between the two modules | Why the board may accept one and not the other | Source |
|---|---|---|
| Module family (UDIMM, RDIMM, LRDIMM) | A board takes one family; Supermicro answers a board that only supports unbuffered DDR3 ECC and non-ECC memory | Supermicro FAQ |
| ECC and non-ECC | ECC support needs processor and chipset support; Lenovo SR665 takes ECC modules only | Intel; Lenovo |
| Density or rank | Processor generation limits: Dell 14G with first generation Xeon Scalable needs 8 GB density | Dell article |
| Capacity | Dell R860 and R960: 96GB DDR5 RDIMMs only with extreme core count processors | Dell article |
| Voltage | Lenovo SR665: different voltages do not coexist | Lenovo |
| Faulty module | One module has failed | Test result |
The examples come from specific Dell, Lenovo and Supermicro systems, so read the rules for your own model. In each case the module may be good and simply unsuited to the system. See how to identify an incompatible RAM module for how to read each attribute from a label or part number.
If the two modules are different on purpose
Mixing a new module with an old one is a common cause. Even where the manual allows mixed capacities or ranks, it sets limits. Dell's R750 allows mixed capacities but not more than two capacities in one system, and Lenovo's SR665 requires the higher density or rank to be installed first. All modules run at the speed of the slowest. See mixing RAM modules for the full table.
If the module is simply faulty
A module that fails alone in a known-good slot, and works in another system, is not a fit problem. Run a memory test where you can. MemTest86 notes that errors do not always mean bad RAM, since the test also exercises the processor, caches and board, so repeat the result with the module alone. For servers, see how to identify a failing server DIMM and when to replace a server memory module.
Next step
If the answer is that the second module is the wrong kind, send us the board or server model and the installed part number. We will confirm a match before you order. Browse DDR4 RDIMM or DDR5 RDIMM, or read memory types explained and the ECC vs non-ECC guide.
Frequently asked questions
Why does my motherboard recognize one RAM module but not the other?
One module may be faulty, the two may differ in type, ECC, density or rank in a way the board does not accept, or the slot may be the problem. Test each module alone in the same slot, then in a second slot.
How do I tell a bad RAM module from a bad slot?
Test each module in each slot. If one module fails in every slot, it is the module. If every module fails in one slot, it is the slot.
Can two RAM modules work alone but not together?
Yes. MemTest86 says errors that appear only with several modules installed often point to a multi-channel configuration problem. Matching modules and a board-listed model help.
Can ECC and non-ECC modules be mixed?
On many systems no. Lenovo's SR665 takes ECC modules only, and Intel notes ECC support needs both processor and chipset support. Check your board and processor.
Does the RAM slot I use matter?
Yes. Boards name the slot to fill first, and MemTest86 notes the same module can behave differently in different slots.
Sources
Read on October 30, 2026: MemTest86 troubleshooting guide, Supermicro support FAQ on a board that accepts only unbuffered memory, Intel article on finding ECC memory support, Lenovo ThinkSystem SR665 memory population rules, Dell PowerEdge R750 Installation and Service Manual, Dell article on 14th generation PowerEdge servers that do not detect new DIMMs and Dell article on the R860 and R960 No memory found error with 96GB DDR5 modules. Rules differ by model and generation, so confirm your exact system in its own manual.
Memory matched to your systems
NEMIX RAM is a US-based memory specialist, in business since 1993. Every module is researched, matched, and compatibility-tested by NEMIX RAM, is JEDEC-compliant, and is backed by a lifetime warranty; register your purchase to activate it. Purchase orders, net terms and volume pricing are available for IT teams, resellers, and government and education buyers. Request a quote with your system models, processors and quantities, or contact our sales team. Browse DDR4 RDIMM, DDR5 RDIMM and LRDIMM, or see our data center server memory page.
NEMIX RAM is an independent memory supplier and is not affiliated with the companies named here. Dell, PowerEdge, Lenovo, ThinkSystem, Supermicro, Intel and MemTest86 are trademarks of their respective owners, used here only to identify sources and compatible systems.