Should I turn off my external hard drive when not in use?

As it turns out, leaving a hard drive running isn't particularly harmful. It may be wasteful, but I'll talk about that below. Many computers have their hard drives running 24 hours a day seven days a week for years. A study by Google a while back concluded, among other things, that if the drive doesn't fail within the first year (so called "infant mortality") it's actually likely to have a relatively long and healthy life.

The "problem" with the Google study was that the drives were left on 24 hours a day, which doesn't reflect common consumer usage.

And here's where things get sticky: it's known that heating and cooling electronics like disk drives repeatedly does cause wear and tear. Exactly the kind of heating and cooling that a drive might experience when you turn it on only when needed. It heats up, you use it, you turn it off it cools down. Over and over again.

So that might make for an argument for leaving it on all the time, right?

Not quite. External hard drives in particular typically "spin down" or turn off after a period of inactivity whether you want them to or not. The next time you access the hard drive it spins up - the cause of the delay you sometimes experience after you haven't used the drive in a while - and then stays running until some time after you stop using it.

In other words, no matter what you choose to do with the external drives, they may already be doing something else.

And of course we need to factor in energy consumption to confuse things even more.

One of the big reasons that drives do spin down when they're not being used is not related to the drives at all, but rather the power that they use. It takes more electricity to keep the drive spinning that it does to just keep the drive electronics ready to go. So by stopping the physical drive you end up using less power. In fact this is often an explicit option in the power settings for laptops, though I know of no way to control it directly in most external drives.

And to be honest, keeping a drive running 24 hours a day if you only use it once a month is wasteful. Which introduces the final complication:

Just how often are you using that drive, anyway?

As you can see there's really no simple answer.

So I'll give you a rule of thumb: for most usage you're probably fine to treat your external hard drive as an integral part of your computer. By that I mean turn it on when you turn your computer on, and turn it off when you turn your computer off.

And because the drives are probably already spinning down on their own, whatever you choose isn't likely to make a huge difference.

As for me, well, I'm an edge case to be sure, but here's what I do: all my computers are on 24 hours a day. I have four external drives that I also leave on all the time. They're all used every night as part of my backup strategy and of course they automatically spin down during the day unless I happen to use them for something else. If they were used less frequently (say once a week) I'd be tempted to turn them off when not in use.