HDD (maybe mobo) freaking out, super slow speeds and hanging boot UPDATE: fixed, see my most recent comment.

I recently built a new PC buying everything new except the storage (1tb samsung 970 evo SSD, 8tb thoshiba something something HDD), the graphics card (rtx 3070) and the ram (32gb crucial lpx). The motherboard is a gigabyte a520i AC with ryzen 7 5700x, and a EVGA 700 br psu. I initially bought a 600w psu, but I quickly realized after some gaming that it wasn't enough for demanding titles. but after replacing the 600w psu for my EVGA 700w, the HDD started being super slow, even hanging my boot (while not even in the boot order), on both windows 10 and linux mint. taking several minutes while booting from a reputable SSD, or a LM live boot from a usb3 flashdrive. Both boot near instantly when the drive is unplugged, and I even used an old doorstop 430w powersupply from an old rig to power the HDD while troubleshooting, determining that the problem was not an under-powered HDD. Can anyone provide some insight? The only help I've been able to get was telling to me to fix my BIOS boot order, but that was my first step.

I have also:

• turned off fastboot in windows
• swapped SATA cables
• externally powered only the HDD, as mentioned before
• tested the same HDD on a different computer, showing correct speeds and actual functionality
• swapped HDD for one of the same brand, showing the same results on this new computer, but expected results on the seperate machine.
• reseated all connectors
• tried turning it off and on again

while booting, linux mint is raising the error blk_update_request: I/O error, dev sda, sector 1953739192 op 0x0: (READ) flags 0x80700 phys_seg 2 prio class 0