Did you notice that even if disk activity is 0% then also it is blinking? If system is accessing the hard disk, even for less data at very small time intervals, the LED is supposed to blink at the frequency with which data being accessed. It will be ON if disk usage is more and for more time it is being performed.
I think this is normal then. As you are watching videos, system is accessing HDD to fetch next video data. It is a small access request sent by OS to HDD, but it needs to be sent very frequently. Because of which you are noticing LED blink.