Asrock releases a PCI 16 5 card for up to 4 mn2
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https://www.asrock.com/mb/spec/product.asp?Model=Blazing Quad M.2 Card#Overview
It does not specify whether it supports RAID.
Utility? No idea.
Back in the day, we already saw hard drives with a PCI E connector
And currently, multiple m.2 to PCI E adapters are sold that are logically "capped" at the speed of the PCI E port.
Back in the day, I looked into mounting an m.2 on a motherboard that didn't have a port and the speeds were terrible.
I suppose that on a Creator motherboard (which usually have more PCIe ports *16 could be interesting for calculations and such.
But for 90% of mortals...
I don't see it.
Regards