RTFM
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Sounds like what you're looking for is an ATX12V plug. It's a 2x2 connector that normally has two yellow and two black wires. It normally goes into the 2x2 receptacle on the motherboard to power the processor. In this case, the eGPU enclosure needs it for some reason, maybe for more power.
The good news is that the 2x4 breakaway connector (called EPS12V I believe) that splits into two 2x2 connectors is probably compatible with this receptacle. One of the two 2x2 pieces of the connector should fit into the eGPU's power receptacle, and the other won't. If it fits, it is probably the right connector. If two of the wires going to that connector are yellow, and two are black, then it's almost certainly the right connector.
You may have multiple of these 2x4 breakaway connectors. If so, they should behave identically, and you can break up any of them and try to fit the pieces into the ATX12V receptacle.
List of ATX power supply connectors: https://en.wikipedia.org/wiki/Power_supply_unit_(computer)#Connectors (without images, unfortunately.)
Don't forget about the big 20-pin or 24-pin main ATX motherboard power connector. Your second power supply, since it is non-modular, will need something to simulate the motherboard's power button. That's can be as simple as a switch between the PS-ON wire (green) and any ground wire (black). But hopefully your eGPU has a place to plug in the ATX motherboard power connector, and handles that on-off switching for you.
Thanks for all the help mate, got it working, see update.
oh bugger. The results were underwhelming to the say the least. NOTHING happens. I was expecting I might have trouble getting things working right away but I must say I wasn't expecting literally nothing to happen.
I have the card in the PCIe slot on the chassis, the ATX PSU screwed in place and the cables from the PSU plugged in to the chassis 24 pin and 4 pin slots. I turned on the chassis' own switch, the PSU's switch and also plugged the thunderbolt cable in and nothing. No fans whirring, no smell of anything frying, no sound indicating I'd just broken 3 expensive pieces of equipment at once, just nothing. Any ideas? The chassis in question is this thing https://peladn.com/products/graphics-card-docking-station-1 and the PCIe card is this thing https://www.blackmagicdesign.com/au/products/decklink/techspecs/W-DLK-32 .
It's all hooked up like this https://imgur.com/a/nothing-happens-PftKNUk
Thanks so much, this is very encouraging. The eGPU chassis has it's own little built in power switch. It also has a 24 pin socket that it needs to power itself so hopefully I don't need to do anything complicated like simulating anything as I'm a little lost by that idea. The power supply itself has a power on/off switch as well.
In the interests of brevity I may have given the wrong idea of the setup. I'm trying to make use of an old Blackmagic Decklink mini-monitor 4k pcie device. I now use a laptop with TB4 ports, rather than the PC this card used to sit it. This card is actually not a GPU, which this eGPU enclosure is supposed to be for, but I'm hoping it will work just the same. It's a TB4 eGPU chassis that takes an ATX power supply and has a single PCIe 16x slot. The card is actually a 4x slot but I think it should work. Anyway there's only the one power supply, this old 600w PSU that I've cannibalised from my old PC.
Those are probably for the cpu but what does the manual for your egpu enclosure say
The manual, entirely in Chinese and only a couple of pages long, says to connect the enclosure to the PSU and the PSU to the GPU. That's all. Although even if it were a more rigorous manual I don't know if they could really provide much guidance here. It's designed for you to use whatever PSU you want, either ATX or SFX and just hook it up.
I'm just asking because I haven't done a lot of PC building before and am not familiar with the ins and outs of types of PSU connectors. In this case, I'm unsure why the PSU manufacturer group these 2 connectors together but gave the customer the option to separate them.
It’s because sometimes the motherboard is cheap/low end and only has the 2x2 header instead of the full 2x4 header, or it has a 2x4 and a 2x2. The little pegs are keyed so see if it physically matches up to what’s on the GPU enclosure
Some systems require more power than a single connector can supply, so they double them up for higher amps.
You should be fine, but pics would be useful.