Why Your Computer’s Hard Drive Lights Flash When You Aren’t Doing Anything

It’s happened to all of us. You step away from your computer and come back a few minutes later. While you were gone, your computer’s hard drive lights start flashing — but what exactly is it doing? It’s natural to be a bit suspicious.

This is generally nothing to worry about. All normally configured Windows systems will do this regularly. Malware is always a possibility, of course. You can run an antimalware scan if you’re worried.

Yes, Your Computer Waits Until You’re Not Around

Your computer probably isn’t trying to be sneaky. Instead, it’s trying to be smart and respectful. Windows has some jobs to do in the background, and it tries to politely wait until your computer is “idle” — when it’s not actively being used by a person — to do these jobs. This ensures the computer’s resources aren’t wasted when you’re using it. The necessary background tasks won’t slow your computer down while you’re using it.

It’s not your imagination — Windows actually waits until your computer is idle to start doing many of these tasks. And, it may even pause the task when you start using your computer again, so if you sit down at the computer to check what’s going on you may see no trace of the activity. The Windows Task Scheduler provides a way to only run a task while the computer is idle, and many tasks are configured to work this way.

What’s It Doing in the Background?

But what exactly is your computer doing in the background? The exact background tasks depend on what software you have on your computer and how it’s configured, but here are some common ones:

  • File Indexing: All modern operating systems include file-indexing services. This is a process that crawls through your entire hard drive, examining each file — and the text inside it — and making a database. When you use your operating system’s search feature, you get instant search results from the database. To do this, the indexing service has to crawl your files and watch them for changes, and this can cause hard disk activity.
  • Disk Defragmentation: Back in Windows 98, you had to close the other programs on your computer before defragmenting your hard drive to ensure it would complete successfully. Modern versions of Windows automatically do any necessary disk-defragmenting in the background, but they’re configured to only do this when the computer is idle.
  • Scheduled Antivirus Scans: Antivirus programs and other security tools are often configured to perform regular, scheduled antivirus scans by default. Your antivirus program could be sorting through your hard drive and examining its files.
  • Backups: If you have automatic backups set up — and you should — your backup utility may be performing a regular backup.
  • Automatic Updates: Windows itself and programs like Google Chrome and Mozilla Firefox all have automatic updaters. If you see your computer busy, it’s possible it’s downloading and possibly installing a new update.

This is just a short list, of course. There’s an almost infinite number of possibilities depending on what software you’re using. For example, if you have Steam open in the background and an update to a game you have installed was just released, Steam will be downloading the update and installing it automatically. File-downloading programs like BitTorrent clients can obviously cause this sort of hard disk activity, too.

Checking What Programs Are Actually Using Your Disk

That’s all well and good in theory, but you might want to know what your computer is actually doing. First of all, if you’re really worried that your computer could have malware on it, you should perform a scan with a reputable antimalware utility instead of just using system tools to see what’s going on. But, if you’d like to monitor your disk activity, you can.

You can use the Task Manager or Resource Monitor tools included with Windows to check per-process disk activity, which is good if your hard drive light is blinking away or your computer is slowing down due to high disk usage and you have no idea why.

To open it, first launch the Task Manager by right-clicking your taskbar and selecting Task Manager or pressing Ctrl+Shift+Escape. On Windows 8, the new Task Manager shows disk activity, so you can just click the Disk header to sort by current disk activity. You can then search for the name of the process to find out what’s going on.

Windows 7 users don’t have this feature in the task manager. If you’re using Windows 7, you’ll need to click over to the Performance tab and click “Open Resource Monitor.” Click the Disk tab in the Resource Monitor window and you’ll see a list of processes you can arrange by their current disk usage. Even on Windows 8 and 8.1, the Resource Monitor window provides more detail than the Task Manager.

To log disk activity and check it later, use Process Monitor — one of the awesome SysInternals tools Windows geeks love so much. You can choose to leave Process Monitor running in the background while you step away from your computer. The next time you come back and see your computer’s hard drive light flashing (and possibly hear a mechanical hard drive grinding away), you can look at your Process Monitor window and check to see which processes were just using the hard drive.

Process Monitor captures a variety of events, but you can click the buttons on the toolbar to ensure it only shows file system events. Below, we can see the Windows search-indexing process was at work.

Process Monitor is good because it provides a history. Even if a process stops using the disk entirely or terminates itself, you still view this information. (Note that you probably wouldn’t want to run this tool all the time, as capturing and logging all system events like this consumes system resources. Process Monitor only logs events while it’s open, so you can’t launch it after a particularly intensive burst of disk activity and see what was happening before the launch.)


Again, this is generally nothing to worry about. All computers will do this, and that’s normal. If you suspect something is amiss, run a scan with an antivirus program. Or, if you’re feeling particularly geeky, look into it yourself with one of the tools above!

Image Credit: Jean-Etienne Minh-Duy Poirrier on Flickr

Chris Hoffman is a technology writer and all-around computer geek. He's as at home using the Linux terminal as he is digging into the Windows registry. Connect with him on Google+.