Amber light on case? What is it?

[finballin]

OK so ive been wondering lately..
What causes the little amber/orange light on the front of computer cases to flash when seemingly in use?
Is it the ram or the cpu?
What causes it?
Every computer has it..

 

[Motopsychojdn]

thats your hard drive activity light that is it flashes when the drive is being used, nothing to worry over
Moto