Which M.2 Slot To Use? – Does It Matter?

The SSD jargon and the technical details surrounding it can confuse even the most seasoned of PC builders. SSDs, PCIe, M.2, Gen3, Gen4, SATA, M-Key etc. are but a few terms that can overwhelm a new SSD stick buyer. And the fact of the matter is that EACH of these details matters tremendously.

These details NOT only matter when buying an SSD stick, but carry an EQUAL value for the M.2 slots that they go into. Oftentimes incompatibility, inferior speeds and hardware conflicts arise when you have not understood which M.2 slot to insert your SSD into. 

M.2 slot is basically the primary slot on your motherboard for plugging in the SSD. However, not ALL M.2 slots are the same. Some M.2 slots are meant ONLY for the inferior SATA SSDs, others suffer from fewer PCIe lane count and / or conform to the older PCIe version.

Motherboards often have multiple M.2 slots which can beg the question “Which M.2 slot to use?” because the fact of the matter is that the choice of M.2 slot certainly can make a difference.

In the following text, I will attempt to clarify all this in detail.

What is SSD? – M.2 SSD and 2.5″ SSDs?

For starters, let me explain what “M.2” means in the context of an SSD.

M.2 is basically a term used to define the FORM FACTOR or the physical shape of the SSD. 

SSDs basically comes in two form factors:

  1. M.2
  2. 2.5″

The stick-like SSDs such as the one you see below is an M.2 SSD. These have a dimension ranging from 22 x 30 mm all the way t o 22 x 110 mm.

Which M.2 Slot to Use
PCIe NVMe M.2 SSD. Source: Pexels.com

The 2.5″ SSDs, however, are much larger in size. The 2.5″ nomenclature essentially comes from the 2.5″ disk platter Hard Disk Drives found on laptops. It indicates the radius of the disk platters.

SATA SSD
2.5″ SATA SSD

So again, the M.2 form factor ITSELF does not entail the performance of the SSD. It just defines the shape of the SSD. What DOES matter in defining the performance, is the interface it carries:

SATA vs PCIe NVMe M.2 SSD Interfaces

There are two main interfaces for connecting SSDs to your PC:

  1. SATA
  2. PCIe NVMe

SATA SSDs

SATA interface is much slower than its PCIe counterpart. The most current version of the SATA interface is 3 and this interface at best can deliver speeds of a mere 550 MB/s for its SSDs.

There are two types of SATA SSDs;

  1. M.2 SATA SSDs – They have 550 MB/s Speeds but go into M.2 slots
  2. 2.5″ SATA SSDs – The one we saw earlier above. These are lager and they connect to your motherboard SATA ports
SATA port labels
2.5″ SATA SSDs connect to SATA Ports

PCIe NVMe SSDs

PCIe NVMe based SSDs use the PCIe protocol and to call this a faster protocol would be an understatement. PCIe Version 3.0 and Version 4.0 SSDs can promise speeds of about 3500 MB/s and 7000 MB/s second respectively.

This is about 7-15 times as fast as a SATA SSD!

PCIe NVMe SSDs can ONLY be found in M.2 form factor.

TL;DR; Both SATA and PCIe NVMe SSDs can be found using the M.2 form factor. However the latter is far more superior and often the one preferred and found on PCs today.

PCIe Version and the Corresponding NVMe M.2 SSD Speeds

As mentioned earlier, NVMe SSDs use the PCIe protocol and are thus affected by its version.

NVMe SSDs use 4x PCIe lanes ideally – PCIe lanes are essentially pipelines that carry information from PCIe SSDs to CPUs.

Versionx1
(GB/s)
x2
(GB/s)
x4
(GB/s)
x8
(GB/s)
x16
(GB/s)
1.00.2500.500 1.0002.0004.000
2.00.5001.0002.0004.0008.000
3.00.9851.9693.9387.87715.754
4.01.9693.9387.87715.75431.508
5.03.9387.87715.75431.50863.015
6.07.87715.75431.50863.015126.031

And with every newer PCIe generation, the per lane bandwidth (speed) doubles! So much so that PCIe 4.0 based NVMe SSDs have DOUBLE the transfer speed as compared to the PCIe 3.0 based NVMe SSDs!

FYI: PCIe 3.0 and PCIe 4.0 NVMe SSDs are also called Gen3 and Gen4 SSDs respectively.

The following table summarizes speeds of different storage drives:

Storage Type
Read Speeds
3.5″ SATA HDD  ~120-200 MB/s (Max)
2.5″ SATA HDD ~80-160 MB/s (Max)
2.5″ SATA SSD ~550 MB/s (Max)
M.2 SATA SSD ~550 MB/s (Max)
M.2 Gen 3 PCIe NVMe SSD ~3500 MB/s (Max)
M.2 Gen 4 PCIe NVMe SSD ~7000 MB/s (Max)
  • The table above shows the theoretical maximum of each drive.

However, in order to install a PCIe 4.0 SSD, you need to have a PCIe 4.0 M.2 slot on your motherboard too:

So, Which M.2 Slot To Use?

With the jargon out of the way, it will be easier for you to understand which M.2 slots to use for your SSD and why.

Check for the Generation

For starters, if you have multiple M.2 SSDs slots on your motherboard, you need to choose the M.2 slot that shares the SAME generation with your SSD at the very least.

So if you have an Gen3 NVMe SSD, you can install it in either PCIe 3.0 or PCIe 4.0 M.2 slot. BUT, if you have a Gen4 NVMe SSD YOU WILL have to install it on a PCIe 4.0 M.2 slot on your motherboard.

If you install a Gen4 SSD on an older slot on your motherboard, the SSD’s performance can be slashed by as much as half its optimal speeds.

(It is also worth noting that installing an older Gen3 SSD into a newer PCIe 4.0 slot WILL NOT result in a performance increase.)

Take for instance the specifications for the ASUS Prime B550M-A motherboard below:

ASUS Prime B550M-A M.2 configuration
Image: ASUS Prime B550M-A M.2 configuration

As you can see, this motherboard has TWO M.2 PCIe slots. One conforms to PCIe 4.0 (M.2_1) and other (coming from the B550 chipset) conforms to the older PCIe 3.0 version (M.2_2).

So in this case, if you had a Gen4 NVMe SSD, you would ideally plug it into the first M.2 slot (M.2_1). If you had a Gen3 NVMe SSD, then it wouldn’t matter which one you use.

Check for the Lane Count

Another very important consideration is the amount of PCIe lanes the M.2 slot on your motherboard connects to.

ASUS X470-F motherboard chipset Specifications 3
Source: ASUS STRIX X470-F Gaming

Recall from earlier that an M.2 NVMe SSD ideally requires 4x PCIe lanes. If an M.2 slot on the motherboard has fewer lanes, then overall speed of the SSD will decrease.

Take from instance, the ASUS Strix X470-F gaming motherboard above. This motherboard has 2 x Gen3 M.2 slot. However, the second M.2 slot ONLY operates in the x2 mode. Meaning, it ONLY connects to 2 x PCIe Lanes.

As such, if you were to install a compatible Gen3 SSD into this slot, then its speed will be HALVED!

Check for the Size

Finally, the size of the SSD stick matters.

M.2 Slot Length
A typical M.2 slot with the different length marked. The holes at different length intervals are where you would screw in your SSD. Source: rog.asus.com

M.2 SSDs come in various sizes and they are labeled as 2230, 2242, 2260, 2280 and 22110.

These numbers tell the dimension of the M.2 SSD stick where the first two numbers correspond to the width in millimeters and the rest of the numbers correspond to the length in millimeters. 

So, a 2230 stick has dimensions of 22mm x 30mm and a 22110 stick has dimensions of 22mm x 110mm. 

While the majority of the M.2 slots on motherboards can take in all sizes of M.2 sticks, there are some that cannot.

Take again the ASUS STRIX X470-F Gaming above for instance. One of the M.2 slots on this motherboard has a maximum supported length of 2280 SSD sticks. Hence this slot will NOT be able to support the 22110 M.2 sticks.

Is the M.2 SSD SATA or PCIe NVMe?

And finally, you have to make sure whether the M.2 slot can even support the interface of your M.2 SSD.

Majority of the M.2 slots on motherboards can support BOTH SATA and PCIe NVMe SSD. However, on few motherboards, particularly the only ones, you may find an M.2 slot supporting only either of the interfaces.

Again, M.2 SATA and NVMe SSDs may both look the same, but they are worlds apart in terms of performance.  You can tell which one is which by reading its labels and also by looking at the connectors (SATA M.2 SSD has two notches, while NVMe SSDs have one notch only).

Samsung Evo 860 SATA SSD m.2
Samsung Evo 860 SATA SSD with M.2 form factor
samsung 980 pro
Samsung 980 Pro. M.2 PCIe 4.0 NVMe SSD

Final Words

To understand which M.2 slot to use on your motherboard, you first have to be well versed with all the jargon surrounding SSD.

Oftentimes you see people complain about inferior speeds only to find out that they have inserted their SSD into an incorrect slot or that their motherboard simply does not have a compatible slot to begin with.

But the bottom line is that the choice of the M.2 slot on your motherboard DOES matter.

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Author:

Atif Qazi
Atif Qazi is the founder of PCGuide101 and an expert in the computer peripheral industry with over two decades of experience. He has worked as a consultant for major companies and has a deep understanding of the inner workings of computer peripherals. He has a degree in Electrical Engineering and has served as a product manager and technical consultant. He is passionate about testing and evaluating the latest products to provide readers with reliable information.

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