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I was wondering: What's the difference between shutting down the computer and shutting down the hard disk?

Aren't they the same thing?

That is: I can't use the computer if the hard disk is shut down. If I shut down the computer, isn't the hard disk shut down along with it too?

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"sleep" saves the current state in RAM, "hibernate" saves the current state to hard disk. Both result in the computer being powered down, but "sleep" uses some battery power, but is quicker to power up again. –  w3d Oct 20 '11 at 10:46
    
@w3d I mean I'm pretty clear about the differences between to sleep and to hibernate, but confused about the RAM and hard disk –  Pacerier Oct 20 '11 at 10:48
    
You can't use the computer if your monitor is turned off, and if you shut down the computer the monitor shuts down along with it too. This doesn't mean that turning off the monitor and turning off the computer is the same thing. –  Andreas Bonini Oct 20 '11 at 14:23
    
@AndreasBonini ok.. though we can of course use our computers even when the monitor is turned off. –  Pacerier Oct 20 '11 at 14:27
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You might find live CDs interesting. You can have the entire OS running normally without using the hard drive at all (and if you can manage to cache the entire CD in memory, you could take that out too). –  Brendan Long Oct 20 '11 at 17:32
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5 Answers

up vote 13 down vote accepted

"Turn Off Hard Disk", is poorly worded in the Windows 7 power properties, the drive is put into "standby", not actually turned off. The drive is "spun-down" to save energy and wear on the motor bearings. On supporting laptops it is put in an even lower standby state. Like ranon said, it occurs when the system or a controller/driver deems that the user is idle for disk activity. It is an independent item, for example the hard drive can go to sleep without the screen saver or screen-off occurring.

If the drive is accessed, the drive wakes back up, The motor spins the platters back up to speed, and then any access to it is returned. You can tell it went to sleep by hearing it spin back up, or because of the seconds of delay you get when trying to do something.


Hibernate is to save the entire present state of operation by moving the ram to disk, and putting the computer into the "off" state.

A computers main Ram needs constant power to keep the Data in it. By shifting the contents of the ram to disk, the contents can be returned to ram. That way even though the ram was empty, the state that the computer was in can be returned mostly. Returning from Hibernate, the contents of the ram are put back in from the hard drive.

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I would have said it is purely to save energy/battery, not wear on the motor. It is my understanding that spinning down and spinning up hard disks can actually increase wear and reduce the life of the hard disk. (?) –  w3d Oct 20 '11 at 11:00
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Yes, getting the momentum going on the flywheel :-) is the hardest load for the motor. there is a balance between bearing wear and motor load. On desktops i like to allow the disk to sleep only if there is not going to be access for a long time. so I set for 45-60 min –  Psycogeek Oct 20 '11 at 11:04
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"turning off the hard disk" means turning it off while your computer is running, but there hasn't been any hard disk access for some time.

Hibernate stores your main memory's contents and cpu state to hard disk and then shuts down your computer completely.

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I mean what's the difference between shutting down the computer and shutting down the hard disk? –  Pacerier Oct 20 '11 at 10:39
    
@Pacerier Shutting down the computer means shutting down the power to, simply put, "all" components. That's why you need to save your RAM contents to the hard drive, otherwise it will be gone. On the other hand, if you just power down the hard drive, the RAM will still be powered. –  slhck Oct 20 '11 at 10:41
    
@slhck Do you mean that we can shutdown the `C:` hard drive and still use the computer? What will happen to the windows stuff under C:\Windows –  Pacerier Oct 20 '11 at 10:47
    
With just the hard disks "turned off" (which obviously saves battery) there will be a noticeable delay when you then use the computer again after a period of inactivity, as the disks need to spin up again. –  w3d Oct 20 '11 at 10:50
    
Usually, your C: hard drive will not get shutdown because there is regular access to it (there are always lots of system processes running, logging, ...). If it gets shutdown, it will be restarted when accessed next - you might feel some little latency as restarting a hard drive takes some few 100 msec. Usually only "data drives" get shutdown. –  Jens Erat Oct 20 '11 at 10:50
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When Windows shuts down a hard drive, it just stops the drive's platters from spinning. The filesystem is not unmounted, although your programs are still running, and the disk's write cache is purged. However, Windows knows that any attempts to access the disk will require it to be powered back up, so you will notice a delay if you try to use the computer after the HDD is shut off.

When you actually go Start -> Shut Down, Windows unloads all running processes, purges all disk write cache, and then unmounts the filesystem. And then your entire computer shuts off.

The big difference between the two is power consumption. Your computer is using more power then in a standby state when the hard drive is shut off (since your CPU and RAM and Motherboard still need full power), although the computer may use significantly less power than it did before. Turning off just the hard drive also allows some tasks to continue running, so long as they don't require hard drive access.

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There's no context specified in the question, but my experience has been that non-technical people often refer to "hard drive" when they actually mean the entire computer. It's highly probable that they are using that particular terminology because the believe that it sounds more technical than just saying "the computer". In reality, while providing technical support, when a user refers to the "hard drive" it's a red flag that that you are dealing with someone very little technical know-how, and you'll have to make sure that any explanations that you give them are simplified as much as possible.

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When computer starts:

  1. OS is loded from the drive onto the memory.
  2. When loading is done, if configured, it can turn off an HDD to save power or reduce noise. An OS operates fine without an HDD as long as everything it needs is located in memory.
  3. HDD is turned back on by the OS whenever an on-disk resource is required.
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Are you saying that theoretically when the OS is already loaded, we can wipe out the HDD since the OS is already in memory? –  Pacerier Oct 20 '11 at 14:17
    
Yes, and the moment the OS will require something from the drive it is most likely to fail. Wiping is only possible theoretically though, since most OS's (there are basically two of a kind) will be constantly spinning the system drive. My Windows 7 machine though seems stop even the system drive sometimes, but I may be wrong. –  Maxim V. Pavlov Oct 20 '11 at 16:12
    
Actually yes you can wipe the drive and it keeps on running. I learned that the hard way. The drive was on the blink but still working. Startup was very slow. Then one day the drive just died but the computer ran for some time until it needed something from the now dead drive. –  John Smith Oct 20 '11 at 16:13
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