LED Connector

Hard disks use a light-emitting diode or LED to indicate drive activity. 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. Of course, this is no longer the case with modern computers (I'm just old).

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.

Hard drive manufacturers long ago abandoned putting the LED on the hard disk itself; it is now integrated into the computer case, which is in turn connected to the motherboard and receives disk activity information from the onboard disk controller.