I upgraded my system from an i7 4790k with 16GB of DDR3 RAM to a 9700x3D with 32GB of DDR5. I tried to use my old Corsair RM850 power supply, which is from the era of my i7. The new motherboard is a Gigabyte Aorus 870E and requires a 24-pin connector plus two 8-pin connectors. I was short one 8-pin, so I borrowed one from a friend's RM750 power supply, and I connected it along with a double 4-pin. When I powered it on, I heard a boot-up sound followed by a fizzling noise. Now, it won't post. Did I just fry my setup?
4 Answers
Yeah, it sounds like your components might be toast. You really should never use cables that didn’t come with your power supply because they’re not standardized. That's a rookie mistake!
If worst comes to worst, what's my best course of action? Are my hard drives ruined too?
It's definitely possible you fried something. Modular power supply cables aren't interchangeable, and using the wrong ones can cause serious damage to your components.
You only really need one 8-pin connector for the CPU power. If you read the manual or did a quick search online, you might have avoided this mess!
Or just watch a video tutorial or ask for help before plugging things in!
Seriously, it could save you a lot of headaches!

You'll want to test each part individually to see what, if anything, got damaged.