So today I finally restarted my computer after something like 20 days and upon booting it just froze on the Lenovo starting screen. Upon trying to fix the problem, I discovered that unplugging my secondary HDD (a 1TB WD Blue) solves the problem.
I've tried out multiple SATA cables, tried all SATA ports on the Motherboard, tried disabling AHCI, nothing helped. I was about to accept the fact that the Hard Drive is fried, when I had an idea. I put the HDD in an USB HDD Docking Station and, surprise surprise, it's working.
I ran Crystal Disk Info to check the HDD, it says Health Status good, temperature is good, and all seems to be running quite smoothly, apart from the performance of course.
I'm now at a crossroads. Could it be the HDD itself or could it really be MB-based? Or am I missing something entirely?
Any help would be much appreciated.
Additional info: The drive is detected in BIOS when connected over SATA. The drive is 4 months old. Never made any problems. Curious fact, possibly related: before restarting (also the reason for the reboot) Windows 10 Start button and search pane were not working.
Update: Tried another driver over SATA aaaaand... it works. Which begs the question: Why is the old one working over the USB Docking Station (which also uses SATA, obviously)? The drive seems to be in perfect condition otherwise.