In practically every computer I've had, there's always been a light to display hard drive activity. What's its origin and original use?

  • To my recollection, going back to the late 60s, there has (almost) always been an activity indicator light on the hard drive. The one case I can recall there not being one was on the HP head-per-track disk in a HP 2000B time share system. In that case, since there was no arm actuator there was nothing to indicate activity for. Commented Sep 6, 2011 at 1:39

7 Answers 7


Well, I can say, as an engineer for MiniScribe (later Maxtor), it's exactly what it appears to be. It's an "activity" light. It was a way for us to tell that a command had been received through the interface and was underway. It was a debugging tool for those of us who, you know, built disk drives.

  • 5
    People build those still? ;)
    – RCIX
    Commented Sep 12, 2010 at 8:21
  • 8
    @RCIX, as opposed to what, growing drives?
    – Synetech
    Commented May 25, 2011 at 4:01
  • 7
    You mean, they arn't delivered by a stork? ;)
    – Journeyman Geek
    Commented May 25, 2011 at 5:16
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    @Synetech: i thought all of our equipment was assembled by nanodevices in vats of mineral-rich fluid!
    – RCIX
    Commented May 26, 2011 at 0:22
  • 3
    Stop this silly talk. We all know that harddrives are baked from the finest ingredients.
    – Jarmund
    Commented Aug 10, 2016 at 13:22

Floppy drives always had activity lights too, as you did not want to accidentally eject the disk while it was still being used.

It was also useful to see if your shell command was being executed as expected. Hard drives just continued to include what everybody was used to having on floppies.

There was a HDD activity light on the 20MB HDD for my Amiga A500.

Also, if you think about early mainframe computers, the only realtime feedback the operator got was all the activity blinkenlights.

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    I also add that if you think about early mainframe computers, the only realtime feedback the operator got was all the activity blinkenlights.
    – paradroid
    Commented Sep 8, 2010 at 7:40
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    Can I +1 a comment?! The word blinkenlights demands an upvote! Commented Sep 8, 2010 at 11:39

Sometimes it's useful to know if the hard drive is active. For example, if the machine isn't responding in any way, but the hard drive is active, it may be that the system is overloaded and swapping physical memory to swap, but hasn't actually crashed. If this is the case, it may be best to wait for the slow operation to finish, rather than rebooting the machine.

These days, Windows is more stable and multi-task scheduling tends to work better, so there are probably fewer cases where it is useful.


Long hard drive operations usually occures when user is waiting for something, for example loading game - when a screen can be black. I think this is some indicator, that have to tell user, that computer doesn't hang up, and some task will be completed after a while.

Also when there is too few ram memory, old computers lose lot of speed and because of swp file and not that stable and complex OS's like today - it can appear to be hanged, it's helpfull to user to tell him that computer is working on some data and be ready soon.


As a side note, and some extra historical information, the HD LED on cases and motherboards is an overall (ie, aggregate) activity indicator for all of the (IDE, SATA, SCSI, etc.) drives in the system except floppies—watch the LED when burning a CD/DVD; it shows the activity for the HDD and the optical drive.

However, as seen below, most hard-drives in the past had a 2-pin connector on the circuit board (and even today, some may have them, though likely without the actual pins in place, thus requiring some soldering) that could be connected to an LED. That means that each hard-drive could have its own activity indicator (optical drives don’t generally have the LED connector). That said, in those days, (consumer) systems were generally limited to two (or at most four) IDE drives anyway.

It was quite cool to have multiple LEDs to show the activity for each drive and was a sort of “mod” at the time.

enter image description here

These days, software is more suited to showing the activity for each drive. enter image description here


Need for Hard-disk LED lights

  • The hard-disk LED normally flickers on when the hard disk is being accessed.
  • Hence 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

  • In older Computer this LED was connected directly to the hard-disk.
  • Now-a-days they connected to the motherboard which is preferred in-case you have more than one hard disk so that the system can activate the light whenever any of them are accessed.

Just to inform that if your hard disk doesn't have an led indicator then you may simply use this software http://www.hddled.com/

  • 5
    $20 for a program that does just as much as a $1 LED and only on Windows? Commented Sep 8, 2010 at 13:18
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    People on the ThinkPad forums have been buying it for their X100e netbooks (very nice for a netbook, but not a proper ThinkPad, like a T or X series). I'd probably get that program if I bought an X100e.
    – paradroid
    Commented Sep 8, 2010 at 15:49
  • 2
    FloatLED ( stone-oakvalley-studios.com/index_software.php ) does the same thing as HddLed and is free
    – tcrosley
    Commented Sep 8, 2010 at 16:58

The disk activity light looks like a cylinder because early hard disk drives for mainframes consisted of large cylinders of platters and corresponding sets of read/write heads. The disk stacks themselves on those drives were designed to be physically interchangeable. Shown here is an IBM 350 disk unit used with the IBM 305 RAMAC computer, which dates back to the 1950s.

IBM 350 disk unit

The main purpose of the light is to allow the user to determine if the system is busy due to disk activity. Before the advent of solid-state drives, long load times for the OS and applications were the norm, and while one could hear the hard disk seeking, the light was a much more reliable indicator of disk activity since sequential I/O is not noticeably noisier than disk idle. If the system seemed stuck but the disk activity light was on, chances were good that the system was just waiting for the disk rather than actually hung.

This still holds true for modern PCs that are configured to boot and run applications from a electromechanical hard drive, but with the increasingly widespread use of solid-state drives which can access data in a matter of microseconds rather than milliseconds, the amount of time the system spends waiting for disk is far less, and the disk activity light is of less value on systems with SSDs. In fact, many newer PCs, especially thin-and-light laptops, omit the light altogether. A more comprehensive way to monitor disk activity is to use Task Manager, which as of Windows 8 can show disk activity.

Even so, the light is still of use to determine if the system is loading data from disk or is completely hung. A computer in normal operation will have intermittent disk accesses; if the system stops responding and there is no disk activity whatsoever for several minutes, chances are very good it needs to be forcibly rebooted. In fact, on my custom-built desktop, there is a disk activity light on the motherboard itself so one could see if the disk(s) are busy (assuming a windowed case) even if the PC is on a table and the case's own disk activity light isn't as readily visible.

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