I understand is a laptop why do u say u cant install another HD remove the one in question with the replacement drive.
I have encountered this type of issue many times before, with diagnostic tests coming out negative only to find out that the HDD is the culprit. Just replace the HDD with another functioning one and you are good to go. There is no need for going professional about it. Hardware like HDD in laptop can behave very funny at times.
Sorry but no 5400 rpm hdd should be your OS drive. Get an SSD. Are your fans working? Maybe the heatsink is dusty causing thermal issues & cpu throttling...
Yep Fans are working properly. I'll get an SSD when I could. By the way how much I TB gonna cost me? which brand is better ? What about drivers ? I heard people crying they could only find nVME drivers for Samsung EVO drives. There are no drivers for other manufactures.
1TB ssd is still expensive mainly samsung evo now u need to check what kind hd connector your machine uses first before u buy anything.