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Hard Disk Drive Activity LED
The most important indicator on the case, this LED normally flickers on when the hard disk is being accessed. It is important in that it gives you a visual indication of how active your system is, and can help ensure that you don't shut off your system while the hard disk is active (which you should never do).
Originally, when PCs had only one hard disk, this LED was connected directly to the hard disk. Newer systems have this LED connected to the motherboard, or the disk controller card (if the motherboard does not have integrated disk controllers, or if an add-in hard disk interface card is being used). This is preferred because if you have more than one hard disk, the system can activate the light whenever any of them are accessed. Larger cases, especially those designed for use as server PCs, will often have multiple hard disk activity LEDs, to allow separate connection of a number of drives.
Next: Speed Indicator LEDs