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I have not done this before -- is there a guide with the process that I can refer to? Quick google research I see I can access the bios by holding “Command,” “Option,” “0” and “F” on boot.
Means you need to remove SPI chip (U6100) and reprogram it with external programmer, like TL866, CH341, or similar.
I suppose you don't have Medusa 2, which can do all the job onboard...
Usually serves for test.
But it will have "dirty" ME region, because comes from other board.
However this way cannot completely discard BIOS issue.
If board starts, 100% is BIOS.
If not, doesn't mean old BIOS is OK.