I know this is an old thread but I was looking for a solution for my server to use both ports and found it.
The NIC is blocked via Ethernet firmware but can be unblocked with Intel EEUPDATE tool..
I purchased a used server .. likely amazon/google.. a rackable systems based on intel s5520hc motherboard both NICs light up but only one shows up in bios as having a mac address and usable for int...
github.com
You can configure this in the EEPROM of the 82575. I had the same issue on my S5520HC from Rackable systems (SGI). You can enable it via eeupdate tool from intel. There is a built in hex editor into the tool if you launch it via eeupdate.exe /gui (I used version 5.24 of the tool that i found on the internet).
You will need a dos bootable usb disk and the EEupdate tool. Start the tool in GUI mode and edit away.
The format of the address in positions of the Hex editor 1100 3322 5544. Example MAC 00AABBCCDDEE would be shown as
AA 00 CC BB EE DD in the hex editor.
You need to make sure the first 6 bytes are the correct address which needs to match the sticker on the NIC port. My board was setup to the 2nd address of the 2 so just one symbol needed to change. There is only one address you enter in the first 6 bytes the second NIC automatically get +1 address.
Per 82575 manual, the location of interest is 10h is where the vendor can disable the port for NIC 2 and location 20h for NIC 1. Since mine was disabled on NIC 2 i copied 2 bytes from Location 20h to 10h to copy the settings which per manual take 16 bits.
Its easy to do in the eeupdate utility and just escape out of editing and the program will ask if you want to save to EEPROM.
After that I have both ports working.