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@Toxic_Flange
To plug GPUs into old Mac Pros you had to use just special cables that were available on AliExpress for pennies, but there was always an option to use a passive converter from Molex connector to power the GPU, using a power board seems like even more trouble — can't you do something similar and draw some power from optical or hard drives?
@Toxic_Flange
It's also important to keep in mind that the PSUs have to pack enough power to power that GPU — server ones usually have plenty reserved, but if your GPU starts drawing more than they can provide under heavy load, you motherboard might get fried 😱