Question New PSU wont work with my friend's pc but does with mine

[AidanClark]

My friend was upgrading his gpu and he needed a new power supply so he got a Corsair rm750e but I don’t think it’s working with his motherboard for some reason. The motherboard seems to be getting power as the fans on his old gpu spin even without any Pcie cables connected to it but at the same time the LED on the motherboard that always turns on with the pc won’t light up nor will the LEDs on his keyboard or mouse. At first we thought the PSU might just be faulty so he got a new one but even that still didn’t work. His old PSU still works perfectly fine with the motherboard which indicates that nothing is broken on the motherboard it just doesn’t seem to with with this PSU I’ve never heard of any such incompatibilities so I’m honestly really confused. Yesterday I went to his house to see if I could notice any reason why it wouldn’t be working because I know a lot more about him than pc’s but I couldn’t get it to work either I then took the PSUs back to my house and tried them with my pc and they are both working perfectly fine. So this is why I’m really confused because both the motherboard and the power supplies work but just not together does anyone have any idea why this might be?

 

[rgd1101]

full system spec?

 

[AidanClark]

full system spec?

Biostar b460 gtq
intel i7 10700
16gb ram
old gpu 1650 super
new gpu 6700xt
1tb ssd dont know the make
New PSU corsair rm750
old PSU was a thermaltake 550 something I can’t remember but I couldn’t even find it on the PSU cultists tier so I think it was really bad

 

[--SID--]

old PSU was a thermaltake 550 something

Is this a modular PSU?

 

[AidanClark]

Is this a modular PSU?

No that one was non modular