Mass-Storage class  M.2   Cards...?  Windows 10 only topic...
I want NVME !

The top reason to put this card in any PC is to make the PC boot faster and the OS run faster and to load apps or games 5 time faster at least.
If doing HDD upgrades, you will be amazed at the peformance increased.
I will not go in to legacy cures for this and now to make old PCs run this device.
There are 3 classes of use here, (not legacy) modern PCs in the last 10 years all have a slot for this card.
Some PCs have 2x M.2 slots but one is for WIFI cards only, RTM read your manual on your PC it tells you all these facts as does the PC spec page.
Some PCs have this "M"  slot but are only x1 lane slow.  If yes, see photo #2 card  x4 solution.

The speed of the M.2 is based on price, the fastest ones are sold for speed (R/W) performance. (there are endless bench marks on the web to prove all that) google it.
Note the cards "M:": type  slot cutout on the edge connecteor on the right, this is  the only correct M.2 card for your PC.
Photo#1, M.2 actual. (random picked not spam)
Most cards are 2280 (the 80 means 80mm long is 3.15"inches long (imperial ) It uses  very tiny metric 2mm screw that PCs sold lack, as in MISSING. My card had some in the box,.
Adapter X4 slot or (x8 or x16) card is x4

I always use  heat-sink(HS) spreader on top of the  cards m.2 chip set, the warm ones need a heat sink. (and case fans too if wise, desktops) Cost of HS is $1.50

The M.2 bus speeds raw can be near X4 (500MB/s each, x4  or 2GB/s that SATA-III can not do)  NVMe drives,  provide write speeds as high as 3500MB/s. That’s 7x over SATA SSDs! (potentials only)

Read the data sheets on any card first before spend cash on any old slug card.
There are 4 card types.
B slots (aka B key) (most times  x2 only speeds) Useless on modern PC.
M slot (key) NVME BINGO  the word key means the slot has a key and is keyed to only allow matching cards to work.  M-key cards are modern, 2012 or newer products use these.
B +M  (2 slots seen) okay too.
The card must be marked or spec'd out for NVME rules. (a BIOS upgrade is best to gain that)

Keep in mind the PC;'s BIOS must support your class of card. NVME is what that mostly is , at this time  year 2021. (last decade)
NVME can run on PC from year Oct. 2012 to now. Windows 8 and newer.
"The biggest advantage of NVMe is its lower latency.
 This is mostly due to a streamlined storage stack and the fact that NVMe requires no register reads to issue a command.
AHCI requires four uncachable register reads per command, which results in ~2.5Ás of additional latency."

A modern PC can boot to CSM legacy-boot or secure boot UEFI. 2012 to 2020. 2021 is NVME UEFI class 3 mandated now.
If UEFI mode fails try CSM legacy mode or go into BIOS and turn off Secure boot (and or Legacy boot , on)
Some folks report cloning the old HDD (or SATA SSD) the  new M.2 and afterwords the new M.2 boots ok,  remove old HDD first) and set boot order correcty in BIOS.

FAQ's?: short.

  1.  Do  not hot swap sticks of m.2 memory, Okay?  Unplug all power feeds, AC and if Laptop AC pack and the DC battery packs first. (then push power button to discharge all cap.'s now)
  2.  Do not try to install the card upside down,   if you try you will blow it to kingdom come. (reverse power  to chips of any kind, will blow them up , in fact LETs THE SMOKE out as we say in tech. land)
  3. Warning  Use ESD methods handling all cards, do not touch the pins for fun, do not fail to ground your left hand on the PCs ground frame , before touching the card or inserting it, even buy a ground wrist trap for $5 and win.
  4. The M.2 card fits only in M.2 slots, never directly in to any PCI-e slot of any kind or age or generation (ISA/AGP/PCI/PCI-e/PCIexpress. ever)  The M.2 card has  a KEY and must match your Slot. B to B or M to M.
  5. To boot any M.2 you need compliant BIOS, MOBO , CPU vastly newer, the above adaptor if the MOBO has no M.2 JACK.
  6. An X4 PCI-e to M.2 Adapter card will fit in any X4/X8 or X16 slot.
  7. Warning some old or  cheap mobos, have less lanes than marked on the mobo or by length of same, I have X8 lots that are wired X4   (some are marked X8 (4) ) The manual covers this, did you RTM ?
  8. Warning many PCs have shared PCI-express lanes, so try all sockets to find best speed.

Here are all the slots used, per wiki.  "M" slots NVME are BINGO on all modern PCs. All others are old relic legacy transition cards like ( mSATA or NGFF)

 (what is NVMe)
Works only on newer 2012 PC's seen here  NVMe it is.

If you want to CLONE the old HDD to M.2, or use free AOMEI partition assistant. click clone, click fast.,done.  C to E: (HDD to M.2)

Free to use Macrium Reflect-7(trial?)

Example #1,  Using real destop PC that already has a real SATA SSD, in place, but no slot for M.2.
The job at hand , legacy PC';s and adding M.2 cards.
This card does long 110mm M.2 and has 2 slots , the M slot and B slot. The car is X2 and X4 lanes, as needed.

The PCE-e to M.2 card adapter $10 big money (LOL)

Best is to do a fresh install W10 (or clone the old drive if W10 is stable)

The operating system must be 64 bit for UEFI support. (quote Intel)
I run mine all in CSM legacy mode with GPT drive formatted.  CSM =Compatability Support Module.

Newer PCs for sure 2021 new, must be UEFI installed as old legacy boots are not gone on new PCs by the UEFI NAZI's. (build PC using mobo and win)
Things that can help the boot:  (and even Windows 10 installed to find the M.2 card.
  • Update your BIOS first for Secure boot UEFI. (or not if you like CSM mode like I do) Watch out the UEFI NAZI's do not allow regression of BIOS  now, no loading older versions allowed NOW! (ouch)
  • If your PC has UEFI if allowed and want CSM boots, turn off SECURE BOOT IN BIOS pages, if found year 2000 back to 2012 (w8)
  • If cloning , be sure to remove the old HDD after  the clone, (or they my go to logger heads) then...
  • The Boot order must be set to M.2 . (if you can not, there will be a dead boot)
  • If fresh installing W10 ,  the installer will see the new m.2 card. if not?, the PC may be too old or BIOS settings are wrong, (try legacy mode CSM, or Secure boot Off in BIOS settings.)

UEFI is seen (in 4 classes 0, 1,2,3)  from year 2012 to now 2021,  the Class 3 is not NAZI mandated, and old BIOS is not banned, only on new PC, OEM. ( IDK there plans)
Class 0 is good old legacy BIOS, class 1, and 2 is best (means you get to choose CSM or UEFI secure boot.)
Class 3 is full blown 2021 , NAZI mode , hard butt mandated !

I have had no problems at all , but I use CSM mode only here. (UEFI secure boot I do not need) 64bit Windows is plenty secure in its own right, run here since July 2015 with zero problems.)
I will never buy or use CLASS3 PC ever . I am too old  2020 PCs here last me until I am dead. YMMV

Age matters, and for NVME for sure.
Ask your OEM PC maker for BIOS support upgrades for NVME support.
I have this card set working on my HP Z600. 2009 built and sold new, it is 12 years and the card with M.2 adapter and boots W10 fast in CSM mode. (with most new BIOS)
Other older PCs that failed are with older South brige chips, Z87 or Z97  but were cured with BIOS updates.
Z87 is year 2013
Z97 is ok, mostly
All chipset back to 8 series from h81 to z97 can work. (but I do not dicate to OEM ever, means ASK THEM)

But you need NVMe UEFI support + CSM to boot modern M.2 cards. Using the
Compatibility Support Module, enabled.(CSM)
UPGRADE TIME?:  (one must replace the MOBO if the BIOS patch was never done to your MOBO like the lest line below shows:
First off , please  upgrade the BIOS
On and ASUS Z97 x99, the April 2017 release announcement says it works on M.2 now..  (see the horror link below) and 2015.

Lots more.... ASUS boards have as seen here , vast)
&  more....
Asus Prime Bxxx, H370,P8h61,h97,h81,B85,h110,A88,f285,H170/270,Q170,H310,H370,Q20,Q170,W87... and more click CSM here.
As old as H81. ask your PC maker for support for sure BIOS upgrades.

I used this MOBO, Z270 prime, I have Z170 based Dell XPS 8900 too runs any NVME card. I am in BIOS pages ,set to expert mode, advanced.

Pick other on both.

CSM or DUAL BOOT BIOS or legacy boot, as some call it.  EXAMPLES BELOW:

CSM looks like this:
This is what you should know (see UEFI rules)(
requires a special mainboard BIOS EFI module,
Here is 1 example of using CSM to run M.2  (aka compatibility Selection mode) 
As you can see the Newer AUSU boards have M.2 settings here, the other boards were upgraded in April 2017  (using EZ-flash)
As you can see there are lots of things here to FIDDLE, and each mother board type, has less or more things here, or nothing at all, to support M.2.... (I mean booting it)
This is clearly a DUAL BOOT BIOS,  Note the CSM mode this is legacy mode.

I like this quote from PC world (Tm)
"Without an NVMe-aware BIOS, you can’t boot from an NVMe drive, though anyone with a x4 PCIe slot or M.2 connector can benefit from employing an NVMe drive as secondary storage.
An NVMe BIOS is not a difficult technical hurdle, but it does require engineering hours and money, so it’s unlikely it will stretch far back into the legacy pool. "

The key word there is UNLIKELY ! Sure nobody (OEM) will upgrade 10,000 old motherboards, even though it is easy.(for them)
UPgrade the BIOS, or ask them, your OEM maker for NVME support.

version 2.  7-1-2018