Question GPU / PSU Issue with new build?

Jan 4, 2021
Hi gurus,
I've done a ton of searching, but this is my first build in like 10 years so sorry if I'm missing some beginner info somewhere.

Here's my issue: my new build (Aorus B450 mobo with 3rd gen ryzen compatibility listed on the box, 1 x 8 DDR4 in slot 1, ryzen 5 3600, corsair cx430m psu, msi gtx 970 gaming) won't get up and running. I've double checked all connections, no CPU pins bent, double checked the power switch connectors for the mobo etc. When I try to power it on, I see one light on my mobo (marked CPU) flash once at the same time I see a light on my GPU flash. Nothing spins or lights up. Except of course when I flip on the power switch, sometimes my RGB RAM lights up a bit. However, when I unplug the GPU power, or remove the GPU altogether, it fires up. I then get a VGA warning light because my setup doesn't support the on-board video (bleeping details).

Does this mean my GPU is bad? I don't know how to troubleshoot from here. The instructions for the mobo are weakly brief.

Long story slightly longer, I had to modify one of the original corsair cables to get it to fit. The GPU requires 6+8 pin input, and I didn't have a second 6+2 or 6 pin power cable. I only had one 6+2 on both eneds, and 1 that was 6+2 on 1 end, but an odd fixed 8-pin on the other end that had 2 spaces unused. So I cut that part of the connector off and figured I was OK because the connector shapes leftover (essentially a 6+2 with the 2 not being there) were the right shape and arrangement. I hope this didn't zap my GPU. Maybe I just need the proper cables from the get-go?

Thank you many times for your generous time to help me out!