I had the same problem and here is my recipe
I have intel S2400EP2 Board and Intel x520-sr2 in the raiser
Simptoms: NIC not visible
As said support "this is a known problem" ))
You must disabled option in BIOS "Default Memory Mapped IO above 4 GB"
Im disabling - rebooting and that is working
But i think that this is not the last problem with intel x520-sr2