My pc restarts without any warning while doing non intensive tasks

[Jera34]

I have been having an issue after installing a new hard drive that would suddenly reboot my pc without any warning. This would happen when I'm mainly just browsing the web or watching a YouTube video. It only happened one time while I was gaming and even then it was not an intensive game. I am not experiencing any overheating, nor have I done any overclocking to my CPU or GPU. I've enabled XMP, but even when I turned it off it would still reboot. I switched the ram around the slots and it would still reboot. My only theory is if it is a PSU problem however I am not entirely sure if it is right. I also tried just removing the new hard drive I've put and still rebooted. 

I bought these parts second hand (except the GPU) which might not help case, however I hope someone would still be able to help me out regardless.

I hope someone is able to find the time to read this, thank you!

My specs are:

CPU: Ryzen 7 3700X
Motherboard: Gigabyte X470 Aorus Ultra Gaming
RAM: 2 x 16GB G.Skill Ripjaws DDR4 3200MHz
Storage: Samsung 970EVO Plus 500GB SSD/1TB WD Blue Hard Drive (New hard drive is just another 1 TB western digital)
GPU: Sapphire Pulse 6600XT
PSU: EVGA Supernova 850W G2 80+ Gold
Case: Corsair 275R Airflow Mid-Tower
WIFI: TP-Link 300Mbps PCIe
Cooler: Noctua NH-U12S
OS: Windows 10 Home 64 bit