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The Basics of Solid State Drives and Modules

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Posted · Original PosterOP

In the years that I have been helping customers and the people here on this and other forums out with computers, I noticed that there has been always quite a bit of confusion around Solid State Drives, the different connectors, formfactors, interfaces and protocols. To help others to understand and to help me to distribute the information necessary to understand the basics of such devices, I am creating this thread.


I want to go over the most common misconceptions, the different connectors, interfaces, formfactors and protocols that are commonly used by Solid State Drives and Modules so that you get a good idea what to look out for when shopping for a new drive or to simply understand the concept behind, let's say M.2 2280, if someone mentions it.




Solid State Drive (SSD), Solid State Module (SSM), (SSC)


You might have noticed that I said Solid State Module a lot, which is where we start.
It is common to divide any solid state storage medium into two categories:

  • Solid State Drives
  • Solid State Modules

Solid State Modules are not encased in a housing and can therfor be mounted without the housing.

Solid State Drives need a housing to be mounted correctly.

Sometimes you will also come across SSCs (Solid State Cards), which is a term that is rarely used for solid state PCI storage devices. Just like a graphics card, it plugs into a PCI or PCI-e slot.


This simple differentation already tells us whether the storage medium can be for example hot-swapped or mounted externally or only internally of a system.

Overall I find it not important to differentiate between SSDs, SSMs and SSCs, because the formfactor alone already tells us everything we need to know. I will mention SSDs and SSMs for continuity throughout the thread.


A good example for this would be a typical 2.5" SSD and a M.2 2280 SSM, as shown in the picture below:







SSDs and SSMs are now further devided into their formfactors. The differentation between SSDs and SSMs does not tell us where the mounting points are located and the actual size of the device.

That is where the formfactor comes into place. Here are the most commonly used formfactors today:


  • SSM
    • M.2
      • 2230
      • 2242
      • 2260
      • 2280
      • 22110
    • MO-300
      • full-size
      • half-size
  • SSD
    • 2.5"
    • 3.5"
    • PCI add-in card (SSC)


As you can see there are quite a few different formfactors and these aren't even all of them, but also only a few of the ones I mentioned are usually used in PCs and servers nowadays.

These are mostly 2.5", M.2 2280, PCI-e and MO-300 devices.


A brief explenation of a few formfactors:



2.5" (inch)


This is still the most commonly used formfactor for SSDs and I'm sure everyone knows it already.

The name for the formfactor is taken from their older HDD (hard disk drive) counterparts, where the spinning platter inside the drive is 2.5" wide in diameter.

Their thickness usually varies between 7mm and 9mm.


M.2 2280

M.2 does not only specify the formfactor, but also the connector, which I will go over later.

The number 2280 consists out of the width and length of the module in millimeters.

That means the the module is 22mm wide and 80mm long.




The PCI or PCI-e formfactor does not specify much. It doesn't tell you the thickness, width or length of the actual device, but rather the necessary measurements for a device to fit into a PCI or PCI-e slot.

A device can use up multiple slots. We see this happening with graphics cards a lot, where cards often take up 2 or 3 slots and vary in lenth and width.




While you probably never heard about MO-300 I am sure you are well aware of M-SATA, which is the connector that is usually fitted to MO-300 devices.

MO-300 comes in two varieties: Full size and half size. One is simply double as long as the other.

They are very similar to M.2 drives, but often not as long but therefor wider.

Usually you will find these devices in either older systems or systems that simply don't need the newer M.2 standard.







SSDs and SSMs connect in many different ways to a device and we will find in this section, that some formfactors reappear in the name of connectors. This is very common and is why this can be so confusing. On top of that I will go into interfaces later, which are not the same as connectors, but also use the same or similar names.


Connectors describe the very physical layout of two interlocking pieces - the dimenstions, the amount of pins and which signal they carry.

They are often also called "hardware interfaces", which I find to be a bit misleading and confusing.


Here are a few examples of the most common connectors:


  • SSM
    • M.2
      • B-Key
      • M-Key
      • A-Key
      • E-Key
      • ...
    • mSATA
  • SSD
    • SATA
    • U.2 (SFF 8639)
    • PCI-e
      • x1
      • x4
      • x8
      • x16


I don't want to explain what each connector looks like and what they're used for. I think everyone can find that out very quickly by simply googling the name of the connectors.

But I want you to provide with a few handy pictures about the different key-layouts of the M.2 formfactor, which you will find below.

To differenciate what the specific M.2 module is used for, the manufacturers seperate the pins by a so called "key", which is nothing more than a gap between the pins at a certain position.

It's very similar to the gap you will find in RAM DIMMs (memory sticks) that indicates whether you're dealing with a DDR3 or DDR4 module for example.



Häufig gestellte Fragen zu SATA-, NVMe- und M.2-SSDs - Kingston ...



Welche Bedeutung haben die Keys von M.2-SSDs? - MEDION Community






There aren't a lot of interfaces used anymore for storage. This is due to the fact, that we don't need a lot to simply read and write data from and to devices.

The most common interfaces are:

  • PCI-e
  • SATA
  • USB


Before I go deeper into interfaces, I want to mention that this is the third time that I list PCI or PCI-e in this thread.
This is also where I want to remind you, that I will go into a specific examples later on, to make very clear how exactly formfactors, connectors and interfaces play together.


Now onto interfaces...


The interface is in the simplest words a guideline to define which signals are needed to get a device to work correctly.

So in theory a PCI-e x4 SSD can not physically connect to a M.2 2280 slot, but they both use the PCI-e interface!

This means that the necessary pins and signals are available to the M.2 2280 SSM for it to communicate the same way as a PCI-e x4 SSD would.

The system does NOT know the difference between the two drives, even tho the connector and formfactor are entirely different.


This is also why USB to SATA adapters for external SSDs are so cheap and work so easily.
USB already carries all the necessary information to interact with a SATA device over the AHC-interface, by supporting both interfaces (USB and AHCI)


It is important to note, that the communication itself is not handled by the interface, but instead by the protocoll, which is the last puzzle-piece to complete the confusion.





The protocol is the necessary software layer inbetween devices to communicate between each other.

It interprets the signal that it is given to by the interface and translates it for other devices to understand.


The most common protocols for SSDs are:

  • NVM-e
  • AHCI

For most users it is simply important to know, that the protocol is specified by its interfaces. The interface decides which protocols are carried.

This means that a SSM in the M.2 2280 formfactor, with a M.2 connector can either use a SATA or NVM-e protocol, depending on wheter it interfaces over PCI-e or AHCI. The M.2 connector type changes accordingly to either a B- or M-key!






Here are a few examples of a real-world SSDs and SSMs:


Samsung 970 Evo (SSM):

  • formfactor
    • M.2 2280
  • connector
    • M.2 M-Key
  • interface
    • PCI-e 3.0 (sometimes the version number of the interface is added)
  • protocol
    • NVM-e


Crucial MX500 (SSD)

  • formfactor
    • 2.5"
  • connector
    • SATA
  • interface
    • SATA
  • protocol
    • AHCI


Crucial MX500 (SSM)

  • formfactor
    • M.2 2280
  • connector
    • M.2 B-M-Key (It has two gaps, to fit into M and B-keyed slots)
  • interface
    • SATA
  • protocol
    • AHCI


Intel 750 (SSD)

  • formfactor
    • 2.5" (15mm in hight, notice that the hight doesn't change the formfactor)
  • connector
    • U.2 (SFF-8639)
  • interface
    • PCI-e 3.0
  • protocol
    • NVM-e




The End (for now)


I really hope that this thread takes a lot of the confusion out of SSDs and SSMs and can be referenced for future questions about the topic.


I'm open to criticism and willing to learn!

I don't want to spread misinformation and I took all the information right out of my memory. It is a collection of information of what I've learned over the years.


While I did a lot of fact-checking, I'm sure that there are still little mistakes here and there.
Please let me know if you find any, so I can correct them!

This does also include linguistic mistakes, since english isn't my first language. 😁



I plan on adding additional information in the future about the different kinds of NAND, controllers and how to properly measure the performance of solid state storage.

But I'm not sure whether I will be keeping up with this promise.


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Posted · Original PosterOP
6 minutes ago, NewMaxx said:

You can check the link in my signature for details.


NVMe & AHCI are the protocols, SATA & PCIe are the interfaces.

I knew I would get this wrong!
Even for me this can be so confusing, because AHCI is literally called AHCInterface.

I will correct it.


I hope I can read through your entire guide soon, so I can maybe add a little information here and there if you don't mind.

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Yes, the terms are often used interchangeably and confusingly, including with "standard" and "protocol." However AHCI and NVMe go together while SATA and PCIe go together.

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For embedded devices, eMMC and UFS interfaces are common, the former being seen often in cheap Windows devices. Though these are pretty irrelevant for desktop builders. 

The pursuit of knowledge for the sake of knowledge.

Forever in search of my reason to exist.

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