I thought I’d close this out by a quick follow up.
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The LSI 9300-16i is now working. I had to remove the latest efi bios from the card for it to see drives and be seen by any of the relevant utilities or OS.
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I bought a Ryzen 5 7600 and now have ECC functioning.
It’s been a painful build this one but I’m sorted and now able to get it configured and holding data.