I have M2 SSD disks on my motherboard, on a PCIe x4 slot, and now on a PCIe x1 slot. Yay! I found a use for those unused old x1 1-channel bandwidth slots on my motherboards.

The speeds of modern M2 disks (top-end Samsung 970/980 series w heatsinks are my favorite, but are relatively expensive) are as follows:

On the motherboard PCI 4.0 = 10x faster than SATA disks

On a PCIe x4 4.0 slot = 4x to 5x faster than SATA disks

On a PCIe x1 4.0 slot = 2x faster than SATA

So, if you're looking for an overall disk speedup or for a way to use those unloved x1 motherboard slots, this is an option for you.  

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