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Sometimes when normally using my PC, it will (seemingly) randomly slow down, and maybe sometimes even freeze for several seconds. During this slow down period, it looks like a (I don't know which drive it is) hard drive is constantly being written to. During the last slow down, I started Windows's Ressource Monitor and found out that the System process was writing up to 10MB/s to a drive (I suspect it's the system drive, C:\, but I don't know for sure).

I'm not doing anything unusual (at least, I don't think I am), and most of the time, it will work normally, but, as I said, it just randomly slows down for some times. Any ideas on what might be causing this and how I can prevent this from happening again?

I just unplugged my secondary hard drive, as I suspected it could be causing the problem. And ever since I unplugged it, this DID NOT happen anymore. I don't know why (hell, I don't even know why the System process would write/read from that drive), the only thing of importance on that drive is Steam. Any ideas?

(I have a triple-core processor and 4GB of RAM. My system drive is a WD Caviar Black 500GB, my secondary, 'data' drive is a Samsung drive, which I don't know the model number of, but I can look it up. I can also post my full PC specs if needed.)

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I had a similar problem to this. Turns out my HDD was failing. After 3-4 months of this happening in the last week I lost 4,094 sectors and the hDD reported to Windows it was about to fail. I actually had problems copying some of the sectors on the HDD. – Ramhound Sep 21 '12 at 11:35
Sometimes reported free space is inaccurate, this will cause all kinds of disc access issues slowing the machine, run a chkdsk /r to resolve this issue. – Moab Sep 21 '12 at 17:54

Sometimes when a Hard Drive is going to die it causes a lot of I/O errors forcing the OS to re-write the datas from the beginning (with a more waste of time). So, even if sometimes is a natural behaviour of those devices, you could try to read out some S.M.A.R.T. values trying to figure out if someting is wrong.

To do this you can use some software, for example: CrystalDiskInfo that will tell you if something's wrong (it reads temps, hours of working, errors and so on).

PS: Defrag once a month at least your units.

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In any case, your hard drive is the slowest component of your computer. If this concerns your boot disk, it could be Windows writing away the system restore information (which is pretty random).
What kind of hard disk is it (5700 RPM disks are kinda slow to use as boot disk) ? And how much RAM memory have you got ? Because it is likely the system is swapping memory to the disk (always very slow).

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My system drive is a 7200rpm WD Caviar Black, and I have got 4GB of RAM. (I also added this information to the question.) I think this drive shouldn't be too slow, though D: – Iniquities of evil men Sep 21 '12 at 10:12
You're right, of course. Shouldn't be an issue. Another thing to check is your system setting for the swap file (System - Advanced - Environment IIRC). Possibly, it is not set as it should (although this is unlikely). – pleinolijf Sep 21 '12 at 10:18
It's set to automatic management of the swap file (for all drives, that is). – Iniquities of evil men Sep 21 '12 at 10:32

Use xperf from the WPT (part of the Windows 8 SDK) to trace disk IO:

Run the script, minimize the CMD Window and when you have the issue again, go back to the cmd, wait 15-20s, press a key to stop logging. Open it with xperfview and look in the Disk IO graph which processes causes the high disk IO.

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