Ok guys, the solution for this is so silly that it's embarrassing. I reopened the laptop's bottom cover to now look at the connection between HDD & MB and noticed that it's a separate PCB connected to MB via flat cable. So, either the HDD Board, the cable between it MB, or MB connector. So, only thing I can try without buying extra components is check connection. For this I had to basically disassemble entire laptop to get to the connection on MB & when I got there
THE CABLE WAS NOT SEATED CORRECTLY
Pushed it in hard, lowered the lever, pulled it a little to make sure it was firmly seated, put PC back together, & it's been working fine now for 2 days in a row. He!
Thanks all for your help anyways. From now on, even in laptops, I'll be checking connections to MB before buying anything.