So recently acquired an RX 580 4GB from a sell/buy High Street kinda place and everything was hunky dory with the machine, specs are 9600k, B360N WiFi motherboard with 16GB RAM and running on EVGA G3 550W PSU. Also picked up R7 370 a few days later for a friend's machine and when it arrived thought I would quickly test it in my system. Booted into BIOS but 9 times out of 10 it would lock up the system before getting into Windows. One time I got as far as installing the AMD driver, it just gave me a 173 error saying no card found. Looked pretty buggered so wrote it off as faulty and went to put the RX 580 back inside. That's where problems began, as the card wouldn't display any image and wouldn't even show BIOS, green LED is on to say power is connected to it and fans spin, but nada. System is still booting and just going back to iGPU and works fine. I ran some Prime95 and even fired up games that Intel UHD 630 can handle like World of Tanks and everything is rock solid. Figured might be an issue with RX 580 too, maybe old mining card that was on its last legs etc so set up a return for both of them. Just as a precaution, managed to borrow a 750 ti from someone (tested this exact card previously in this machine) and to my surprise having same issue, green LED on for power, fans spin but zero display, not even into BIOS, machine still works via iGPU though. I cleared my CMOS, switched over the GPU cable (modular PSU) and tried second input on the power supply as well. BIOS is set to check PCI-E first too before switching to iGPU and I can't see either card in the Device Manager. Motherboard is ITX so don't have any other PCI-E slots to try. Checked for any loose bolts in case there's a short somewhere and inspected the slot, don't see any damage of any kind on it or dust inside. Confusing part though is that if I put the R7 370 back in after all this, it boots into BIOS just fine but does its system crashing thing. RX 580 and 750 ti don't display any image whatsoever though so not sure if it can be a completely dead PCI-E slot. At the same time if my PSU somehow got fried, wouldn't the whole thing fry and simply not work? Person I borrowed 750 ti from only has a 400W PSU so can't test RX580 in there and frankly at this point I would be very cautious about sticking R7 370 in there or any other machine. I will be returning the 750 ti (which was working as of this afternoon) just to see that somehow didn't get buggered now but I'm frankly a bit confused as to what to do at this point or what's at fault. To make matters interesting, I picked up an R9 Fury from MM which should be arriving in a few days but guessing there's little change of getting that going at this point.