I'm trying to figure out why both of my Nvidia cards prevent booting the Amiga.
With either an old TNT M64 or a fresh from the box 6200, the Amiga simply won't boot. A PC sees them with no problems.
I've seen the same result in both a Mediator 4000 and a Prometheus.
Is this a symptom of a 3.3v card in a 5v slot or what? It's just too weird that all the other PCI cards I have seem to work, but not these Nvidias.
PCI slots and cards are either 3.3v only, 5v only, or universal. A 3.3v card won't fit into a 5v slot (unless you put it in backwards). All PC's have 5v slots, so all cards for PC's are either 5v only or universal. I don't know about the Prometheus, but I think on the Mediator they decided to not provide 3.3v on the PCI slots even though it should be present even on 5v only slots. So it is possible that the problems you are having are because the cards are expecting 3.3v and not getting it.