Question What's the point of multiple M.2 slots on a MoBo if they eat into PCIe lanes?

[Mick-e]

I wasn't sure if this was a storage question of motherboard question.
I'm looking at building a new CAD rig, and I'm driving myself nuts with configuration options.
I'm set on an X870E board, and yes I know they are almost pointless compared to X670E's but I'm trying to future proof the rig as much as possible as I can usually milk 6 years out of a machine, and maybe the newer chipset gets me a little further down the line.
This amazing document has all of the (most) AMD boards compared
https://docs.google.com/spreadsheet...9O-y_6hv8sPs/edit?gid=755628141#gid=755628141
I've talked myself out of running a RAID1 since the only reason I want to do that is to backup my system drive (because I've had SSDs fail) but I don't need RAID to do that I just need to make a disk image back up, and I found software to schedule that.
From what I can tell with all of the X870, and X670 chipsets as soon as you add a second M.2 SSD you start to eat into PCIe lanes and different boards at different rates.
Or at least that is true for the 5.0x4 mode.
So in this example

ROG STRIX X870E-E GAMING WIFI | Gaming motherboards｜ROG - Republic of Gamers｜ROG Global

The ROG Strix X870E-E Gaming WiFi elevates builds with 18 power stages, DDR5 slots and extensive PCIe 5.0 connectivity. It features five M.2 SSD slots with heatsinks, USB4® support, WiFi 7, AI Overclo

rog.asus.com

Where this board has 3 5.0x4 M.2 slots.
What is the point of having those slots if you lose half of the PCIe lanes?
Again using this board as a reference it looks like slots M.2 slots 4 and 5 are 4.0x4 and don't share the PCIe lanes .
If I were to use the linked ROG then the best scenario is to put a primary OS drive in M.2 5.0x4 Slot 1, a disk image drive in M.2 4.0x4 in Slot 4, and a storage drive in M.2 4.0x4 Slot 5.

What I'm trying to do is have a machine that is as fast as possible for the CAD software to work with OS and files stored. And hopefully have this be good enough for a few years to come.

Am I over thinking this?

 

[USAFRet]

Your "disk image" drive really does not need to be on a fast M.2 port.

Done correctly, this is all automated, in the wee hours and in the background, when you're not using the system.
My Images happen overnight, to spinning drive in my NAS, across the network.

 

[JohnBonhamsGhost]

Where this board has 3 5.0x4 M.2 slots.
What is the point of having those slots if you lose half of the PCIe lanes?

using this board,
if you're only interested in the fastest drives available then you likely don't care about having all PCIe slots populated with all lanes available.

but,
for gaming you'd see no difference vs SATA 6Gb/s and those can be had much cheaper.
so why use up the PCIe lanes with another M.2 drive?

 

[CountMike]

What's the point of number of PCIe lanes if they are not used?
in case of having to choose between PCIe and M.2 slots,more the merrier so you have a choice which is better than not having it. in case some have to be shared, so be it again you have a choice.