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For example, after booting Windows and everything else, when the computer is idle. No file copying or transferring, nor any activity like browsing websites. Does the HDD drive keep spinning continuously in this state?

Would the HDD just slow down the spinning speed and keep running? Or would it stop spinning at all until next user action?

When I close the screen to put the the laptop into Sleep mode, how long does it take for the mechanical HDD inside to spin down? in other words how long should I wait before it's safe to bring the laptop elsewhere?

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    Most HDD manufactures have a energy saving brand that during inactivity the disk spins down. So the real answer to your question is: it depends on the disk
    – Ramhound
    Oct 23, 2015 at 11:07

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Most Windows systems automatically power down HDDs after a period of inactivity. This saves power, and possibly reduces wear on the drive. The problem is that spinning a mechanical disk back up to full speed takes a few seconds, during which the drive is essentially unusable, so that can slow things down a bit when use of the system resumes.

To see what your PC's current settings regarding drive inactivity are, open "Power Settings" (on a desktop you can search for it from Start or in the Control Panel; on a laptop you should be able to right-click the battery/plug icon in the system tray). Click "Change Plan Settings" for your current power plan, then click "Change advanced power settings". One option near the top should be "Hard Disk". Expand that to see (and set) how long the HDD will wait between when it was last accessed and when it spins down to save power.

On a Win10 desktop, the default seems to be 10 minutes. That seems pretty good to me. I have an SSD that I boot off of, so the data disk isn't needed most of the time and can safely go idle and stop spinning.

Some HDDs might slow down to a lower speed rather than stopping entirely, but I think they generally stop spinning entirely once the idle threshold is reached.

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  • On any personal computer (especially a Windows computer), the drive housing the operating system will never spin down. Modern operating systems rely on a stored paging file to handle virtual memory, and this will keep the disk spinning (because it always busy).
    – David Vernon
    Oct 23, 2015 at 11:10
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    @DavidVernon: Incorrect. A pagefile is not always busy and a system drive can and will spin down when idle.
    – qasdfdsaq
    Oct 26, 2015 at 12:34
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It depends on the drive used and the configuration of the operating system. Some hard drives will automatically spin down after a predefined period of inactivity, particularly those intended to save energy. Most operating systems can also be set to spin down a hard drive that has not been used for a predefined period of inactivity.

Whether or not the hard drive spins constantly depends thus on:

  • If the hard drive has been configured to automatically spin down after a set period of time
  • If the period of time after which the hard drive has been configured to automatically spin down ever elapses during normal use

For example, setting the period of time before the hard drive spins down to 30 minutes means that it is less likely to spin down than if it is set to 5 minutes.

It might also be worth noting that spinning a hard drive up and down repeatedly can place more wear on it than leaving it running constantly.

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There are two aspects to your question. Let's take them in turn.

In the general case,

does a mechanical hard drive (not SSD) spinning ALWAYS when the computer is powered ON?

No. There are a number of situations that can cause a mechanical hard drive to spin down. For a trivial example, consider a system with two rotational HDDs, with one of those as the system startup drive and the other used for other purposes (backups, maybe? because backups are always good.)

If you open the computer case and remove the power cable from the second drive (which is allowed by modern interfaces; it's termed "hot removal" in the SATA standard), then pretty obviously the second drive will no longer be spinning, since there is no electrical power for the drive motor, yet the system is still running. Hence, we have shown that "a mechanical hard drive" does not necessarily "[always spin] when the computer is powered on".

On the other hand, the question you probably meant to ask,

in a system with a single mechanical hard drive, is it possible for the hard drive to spin down while the system is working?

which we can answer with yes. The hard drive only needs to be spinning while there is I/O activity occuring on it. Because spinning up takes some non-negligible amount of time (usually on the order of seconds), we generally want to keep the hard drive spinning for a while after the most recent disk activity. However, for reasons of power consumption (rotational hard drives need some 5-10 watts of power just to keep the platters spinning; the exact figure varies, and depends on several different factors), we may want to spin down the drive after a while and accept that the next I/O request is going to take longer to complete.

Different operating systems expose this functionality in different ways, but just about anything reasonably modern will have some way of setting these power savings settings. For example, in modern versions of Windows, you can go into Control Panel and then into Power Options where you can set the hard disk spindown time.

If the system doesn't need to access the hard drive for normal operations, then it's perfectly possible for it to boot from a HDD, take whatever preparatory steps may be necessary, and then allow the drive to spin down while working with data in RAM only, with the hard disk spinning up only very occasionally if indeed at all. An example of such a system could be one that is running number-crunching applications that don't require a large amount of storage.

Modern operating systems, including Windows, often use swap space to both allow applications to allocate and use more memory than the physically installed RAM, as well as to make more efficient use of the much faster RAM. This can cause disk accesses even when those would not be expected, and thus prevent the disk from spinning down. Completely disabling swap may however have unintended side effects, as also explained here. You should also keep in mind that many hard disk drive failures happen during spin-up, so having the disk spin up and down repeatedly may increase the probability of it failing during use.

It's even possible, though I'm not sure modern versions of Windows supports that at all, to build systems that have no local persistent storage capability whatsoever, getting everything they need from another host on the network. Compare for example PXE or Preboot Execution Environment.

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