My PC case's red light

[adr123]

Im installing my win 10, and my computer had to restart, when it started booting up, it sounded as if the computer was working under a heavy load, and this red light appeard. I just want to know what it means

 

[geofelt]

I think that might be the hard drive/ssd activity led.
But, the symbol above looks strange.

 

[ss202sl]

looks like HDD activity light. Maybe a antivirus scan is running.