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Hard disks use a light-emitting diode or LED to indicate drive activity (if you are unfamiliar with LEDs, see the page on basic electrical components for some background information). The hard disk activity LED is a very useful indicator that generally tells the PC user at a glance when the system is active. The first PC hard disks shipped with a faceplate (or bezel) on the front. The hard disk was mounted into an external hard drive bay (in place of a floppy disk drive) and its integral LED was visible from the front of the PC, because the drive's front was actually protruding from the case, much as floppy disk drives still do.
It was quickly realized that having the disks mounted internally to the case made more sense than using external drive bays, but the LED was still desirable. So a remote LED was mounted to the case and a wire run to a two-pin connector on the hard disk itself. This system worked fine when there was just one hard disk, but became a problem in systems that had two or three hard disks. Eventually, the case LED was made to connect to the hard disk controller instead, to show activity on any of the hard disks that were managed by the controller.
Modern PCs have integrated IDE/ATA controllers built into the chipset on the motherboard, so the LED is usually connected to special pins on the motherboard itself. For systems that use add-in controllers, the LED is connected to the controller, as it was in the days before integrated controllers. Over time, as connecting the LED to the controller has become the standard, most manufacturers have dropped entirely the LED connector on the disk itself on IDE/ATA drives.
Since support for SCSI drives is not present in the vast majority of PC motherboards, they often do still come with an external LED connector.
Next: Hard Disk Logic Board