SSDs come in a variety of shapes and sizes. The biggest size is the 2.5" drives that are installed inside desktop computers.
Then comes the smaller and bare looking M.2 SSDs that are installed inside laptops and even on desktops.
There are a lot of technical parameters that describe M.2 SSD drives about how they work and how powerful they are.
Someone made a nice graphical illustration explaining various kinds of SSDs along with the underlying technologies they use. It was posted on reddit here.
Modern laptops, Mini PCs are compact in size and they use the smallest form factor SSDs which is the M.2
M.2 defines the form factor and the slot types used for these SSDs.
As you can see in the above image they look like bare RAM sticks with all the circuitry exposed.
They are small in physical size but not in storage capacity. They are installed directly in their M.2 connector socket somewhere on the motherboard.
So there are the following types:
1. 2.5" (form factor) SSD drives - connected using SATA cables to SATA connectors
2. M.2 (form factor) SATA drive - small ram like stick that are connected to an M.2 slot that support SATA. Mostly used in older laptops
3. M.2 (form factor) PCI-E drive - small ram like stick that are connected to an M.2 slot that supports PCI-E.
4. M.2 (form factor) NVME - same as M.2 PCIe, but uses newer nvme protocol which provides faster speeds. Used by most modern laptops as well as nvme supporting motherboards for desktops
1. The simple 2.5" SATA ssds
This is the old style ssds we all are familiar with. Available from numerous brands including Crucial, Western Digital, Samsung, Kingston, ADATA etc.
Available in sizes from 120GB to 1 TB and even higher, they are used primarily in desktop pcs as primary storage.
They are connected to motherboard using the sata interface cables which have a speed limit of 600 MB/s. This is slowed than m.2 ssds using pcie.
Most recent laptops and desktops are now moving to the newer nvme ssds which are way faster than these traditional ssds.
Several companies like Samsung have also launched the portable ssds which are similar to the 2.5" ssds but can connect to a system through a usb cable. However these externally connected ssds are even slower.
2. SATA M.2 vs PCIe M.2
The first technical parameter about M.2 SSD drives that needs to be understood is the underlying connection technology. It can be either SATA or PCIe.
Note that PCIe over here does not mean PCIe slots as you would see on a desktop motherboard. Its the inner circuitry that communicates data between the motherboard and the drive.
Similarly SATA over here does not mean a SATA cable port as you see on motherboards. Its the inner circuitry.
So inside a laptop you can have a M.2 connector interface that uses SATA or PCIe or both underneath. However an SSD will either be M.2 Sata or M.2 PCIe, but not both.
For example a PCIe M.2 SSD would be incompatible with a SATA only M.2 socket/port.
PCIe M.2 is much faster than SATA M.2. SATA specifications support maximum speed of upto 600 MB/s whereas PCIe (gen 2) can go as high as 2000 MB/s and PCIe Gen 4 can go upto 4000 MB/s.
With NVMe PCIe M.2 goes even higher than that. More on that later.
Which one to buy for Laptop
If you are buying a Laptop then go for PCIe M.2 storage compared to SATA M.2 or a HDD/SATA 2.5" SSD.
M.2 is more expensive but is considerably faster and has superior performance in many situations. Furthermore go for NVMe PCIe M.2 SSD which is the fastest currently.
Faster storage devices enable faster boot times, faster shutdown times, faster copying and moving of files across drives, faster file access time allowing applications to launch quick.
It is worth knowing that some older laptops used the mSATA based ssds which is a predecessor of M.2 interface.
Which one to buy for PC
If you are building a desktop pc, then you have multiple options. Now a days there are desktop motherboards that have the M.2 socket/port allowing to connect M.2 SSDs right on the motherboard without having to use a SATA cable like before.
Alternatively you also install PCIe Card based M.2 SSD on motherboards that do not have a dedicated M.2 Slot/port. They plug into the PCIe card slots on the motherboards just like a Graphics Card.
If you want super high storage drive speeds then you might want to install M.2 SSD or NVMe M.2 SSD on your PC as well.
But remember that that compatible motherboards would be more expensive.
It may not be worth the investment unless you are sure that you need super high disk read/write speeds.
3. NVMe - Improved M.2 PCI-E
NVMe is an improved version of PCIe M.2. Its a new protocol that allows PCIe based SSD to perform even faster.
Typical nvme ssds on laptops and desktops can reach speeds upto 3 Gigabytes/sec. This is way higher than ssd drives connected using sata cables which can go only upto 600 Megabytes/sec.
In simple words, nvme speeds are unmatched by any other technology and it is the future of fast storage on all kinds of computing devices.
At present NVME drives have already become a standard in both laptops and desktops. Most laptops offer around 128GB-1TB NVME storages.
Whereas desktops have the option to use a motherboard with m.2 slot where in a nvme ssd can be installed.
Resources and Links
Kingston website has a page explaining details about SSD, M.2, SATA, mSATA and PCIe
Crucial also has an article on their website explaining stuff about PCIe, SATA and M.2