Inspiron 7373 second hd

The system supports only one M.2 SSD -- the other M.2 slot contains the wireless (WLAN) card.

I see so the 1st M.2 SSD slot takes one PCIe or SATA drive, can another M.2 SSD replace the WLAN card ?

You'd have to try it to see, but:

1.  Even if the drive does work, it'll be storage-only -- not as a boot device.

2.  The drive will run much slower than it should -- the WLAN slot isn't wired for high performance, due to the low bandwidth nature of a network interface.

3.  You may face a capacity restriction due to the limit on the physical size of the card that will fit.

Similar designs as in OptiPlex desktops, both types of SSD drives in your system are using the same M.2 slot.  If you switch out the Samsung SATA drive for a larger capacity NVMe drive, it will improve the system performance (up to x5 times faster in read/write speed) and is solving the concern for adding the second drive.

Yes but can only use a very short hard drive here.

1. Why can any drive put in the WLAN slot not be used as a boot device ?

2. I see then will only use it for storage as boot up is going to be slow. What interface is the M.2 slot for the WLAN using then if has a low bandwidth ?

3. Yes no way to fit an M.2 2280 here but maybe a 2230 is going to work but that looks unlikely 

Sure it looks like the one M.2 SSD slot can support just one SATA or NVMe/PCIe drive and not much can be done with that. I did not know NVMe/PCIe SSD can be up to 5x times faster than SATA. But it is only one drive and not two so will not solve anything as far as that goes. I'm looking to add two hard drives but that cannot be.

1.  Because the firmware isn't written to allow booting from the WLAN socket.

2.  In designing a mainboard, engineers have only so much bandwidth to assign -- lower demand peripherals get less bandwidth.

Current NVMe drives can be many times faster than that, but this system isn't current -- though NVMe is still faster, there are plenty of drives that can outstrip the NVMe interface on this system.

Re:  Yes but can only use a very short hard drive here.

No, can't use a hard drive in that slot, no matter how short.  Because SSD is M keyed and B/M keyed but the WLAN slot is knotted for A/E keyed.  It was designed so that only correct type of device can fit in there.

Beside the key being knotted, the pin assignment for electrical and data are also different between each device.  That was the reason why some M.2 for SATA doesn't work with M.2 for NVMe and vice versa. 

You are insisting on beating the dead horse here.

Key identify between different M.2.  Can you see the difference between the connectors.

M.2 slot for WLAN and M.2 slot for SSD on motherboard.  Can you see they are knotted differently in the slots.

1. I see and nothing can be done to rewrite the firmware so we can forget this

2. Sure in that case it makes sense to add more bandwidth to the hard disk controller and less goes to WLAN, WWAN, and bluetooth as well.

3. Yes this laptop is from late 2017 to early 2018 at the latest so NVMe speeds have multiplied vastly since that time. Not sure what you mean by plenty of drives that can outstrip the NVMe interface on this system, but will go with that.

Well, I can't say that this surprises me too much or even at all. I see so the WLAN interface is different from the SSD one then no way it can work anymore. Yes that is what it looks like to me also. 

Of course if the pins for electrical and data are in a different location for the SSD and WLAN no way it will work. Yes M.2 SATA and NVMe do not go together. 

Not really just asking whether it can be done or not.

Although the image is a bit blurry I can make out the difference in the connectors

This one is a nice picture and I can see the knot is more towards the left side for M.2 SSD slot. Not much else can be said on this matter.