You are spot on with both theories: I updated the BIOS from an old 48 version BIOS with the blue-screen-type legacy BIOS. Theory 2, the mainboard simply cannot supply the resources to run the card, be it firmware or just slot power related issue. get into windows, you might have a screen.or. So theory 1, The board is posting and getting stuck on F1 or Del to continue yada yada. The UEFI bios could be more of an add on to the Legacy where some boards started new with legacy and UEFI later added in. The board is PCIE 2.0 to start with, and likely designed for cards of 75w or less PCIE powered, no external power. What other GPUs have you tested with that motherboard? I remember something about legacy bios modes on GPUs and issues with older motherboards, but the 290 and H61 are close enough in age, so that shouldn't be the issue. Unless someone has any idea as to why the r9 290 won't be recognized by it?

I really have no other option to try than to try to reinstall the bios or downgrade to the pre-gui version bios. Other gpus work with that DH61WW but they're older so maybe the bios didn't add support for it? The motherboard has to be the problem because the r9 290 works on another pc. So I thought that maybe something got damaged back when I flashed the non official bios (probably official, but not directly from Intel). But now I tried to plug an R9 290 but it is not recognized by it. When I updated to version 116 the PC didn't post for quite a while. The problem was that Intel removed all motherboard related support from their website so I had to download some massive torrent file of someone from reddit who actually downloaded almost every file regarding any Intel motherboard before they deleted it. I updated my old Intel DH61WW to version 116.

Dh61ww cpu support