You can understand if it is the fan by tracing your fingers around the sides of the laptop where a grill-like space/hole is usually found. You should feel a gentle breeze.

If it is the HDD that is "working" you should be able to see the HDD light blinking or hear the faint click-clack the head makes when moving back and forth.

If the HDD is working constantly it may be trying to remedy an error it found or installing updates in the background, or doing a scheduled virus scan or whatever and nothing will ever happen. But stay on the safe side and do a complete backup, most external USB hard drives have a kind of "complete backup" or "ghost" program in the box.

I noticed that the laptop is "about two years old". Why don't you check if the warranty has not expired? The Toshibas in my country have a "no questions asked" warranty policy, like if you are bored with it, you drop it on the pavement and they give you a new one, no questions asked.