ASUS Z490 + Intel Pentium Gold G6405 + AMD cards not working

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Hi,

could anyone help to discuss the reasons why my rig doesn’t boot, please?
Configuration:
MB ASUS Prime Z490-P 6xPCIe + 2xm.2 (latest BIOS)
CPU Intel Pentium G6405
8GB DDR4
Cards:
5x Sapphire Radeon Pulse+ RX6800XT
1x AMD RX6800
1x Sapphire Radeon Pulse+ RX5700XT
1x Sapphire Radeon Pulse+ RX5600XT
2x Corsair RM850

The problem. All cards are working fine BUT the rig doesn’t boot when there are more then 4 cards connected to the MB. It doesn’t matter which cards I connect. Any 4xcards combination is working.
Risers seem to be fine. All connectors are properly connected, risers are v009s so they are displaying when the connection is ok.
When I connect any 5th card (PCIe) the rig doesn’t boot. Everything seems fine (mb on, all leds on, cards’ lights on, fans spinning) but the display (connected to iGPU via HDMI) doesn’t turn on.
After I turn the machine on and disconnect the card it’s booting ok again.

I have another rig with same MB ASUS Z490-P, same CPU with 8x nvidia cards and that one is stable and running nicely all the time.

What might be a problem? What am I doing wrong?

Any help appreciated.
Thank you.

I got the same mb and connected 8 cards to it with 0 problems, now using 7 because no room for the 8 one. the only change I made was changing the pcie gen to version 1 instead of auto

I am pretty sure I tried that. But today I saw some more settings I can try like disable VT-d etc.
4G encoding is enabled, I disabled unused devices like HD audio, SATA (booting from flash drive), serial port.
Anyway, thanks for sharing, I know there is a way just need to find it.

I am using the sata with the ssd I have

I got it working yesterday by disabling VT-d and some more setting in BIOS setup.
Now running 2x5600XT, 6800, 6800XT. 2 cards via m.2 → PCIe adapters.
Still need to tweak the OC a bit, hoping for more effectivenes.