The first thing to try is to download the latest PREBOOT utility from Intel and see if it can see the card and install the combo ROM (normally UEFI and PXE). That probably won't solve the problem of the card not getting link, though - the combo ROM is what the card exposes to the host CPU, not the card's internal operating firmware. That is called NVM and as far as I can see, Intel has never released a general-distribution NVM update for the X520. You'd likely need to find someone who had a TIGRIS account in order to find the image. You may have better luck finding the OEM diagnostic tool, CELO.EXE (DOS version) or celow64e.exe (Windows 64-bit). That will at least tell you if it thinks the card is bricked or not:Anyone have helpful advice for how to proceed about unbricking it and downgrading it to something that works?
Intel (R) PRO Adapter Using: Intel (R) PRO Network Connections SDK v2.25.8 Intel (R) PRO Adapter End of Line Diagnostic Utility v1.25.08.00 Copyright (C) 2002 - 2015 by Intel (R) Corporation. Intel (R) Confidential and not for general distribution. Found Intel(R) 82567LM-3 Gigabit Network Connection Card 1: Bus=0 Dev=25 Function=0 MAC Address=0026B9xxxxxx IRQ=160 Testing adapter...hit <ESC> to abort. Device Registers...PASSED EEPROM Test...PASSED FIFO Test...PASSED Internal (Extended) Loopback Test...PASSED Testing completed.