Why does copying a large file sometimes slow down or fail?

Hard Disk Errors

The very first thing that comes to mind is a hard disk issue. Not a failure, per se, and perhaps not even something that Windows will notice.

What can happen is that the magnetic media of a hard disk can become "slightly" damaged. I say slightly, because even though it's damaged, the disk drive itself can actually still read and/or write the data if it tries hard enough.

The reason that this comes to mind is that "hard enough" usually means trying again and again and again until it works. That takes time, and that can manifest as an unexplained slowdown.

Running chkdsk (with the "/r" repair option) might help, but in all honesty probably not. Chkdsk, like Windows, doesn't really operate at the low level that's required to really address this type of failure. SpinRite, which is not free, does.

So, even though it came to mind first, I'd save purchasing SpinRite until we've ruled out other options (unless, of course, you already own it).

How do I fix errors on my hard disk? has more ideas on this direction of investigation.

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Free Space and Fragmentation

An exceptionally fragmented disk, or a disk that is nearly full can often also show signs of slowing down. Defragging regularly - perhaps once a week - and taking care to stay on top of disk usage to make sure that you have lots of free space on the drive are the obvious ways to avoid this issue.

How should I keep my hard drive clean and working at its best? has more tips and tricks on keeping your hard drive running smoothly.

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Software

Another reason that things could potentially slow down is simply the amount of other software running on your machine. Now, a gigabyte should be plenty for WIndows XP to run quite happily, but if you have lots of other things running at the same time it's possible for that to have an impact.

The most common scenario is disk access. If your disk light is flashing continuously before you even start the copy, then something is already accessing the hard disk, and that's going to slow down your disk access. Sometimes it's hard to determine exactly what programs are accessing the hard disk, but tools like FileMon, or Process Monitor can help diagnose.

The copy program itself can sometimes be "at fault", in a way, if you're low on RAM. Some copy programs work by first reading the entire file into memory, and then writing the entire file to its destination. 10 megabytes seems like a relatively small file as files go, but if your system is low on memory that could be enough to cause the system to start swapping memory to disk, which in turn would interfere with the very disk copying operation you're attempting to perform.

You can use process explorer to monitor your computer's memory usage file you're copying files to see if this might be the case.

(As a side note, there is a specific and very similar memory related problem and error message that can often result when copying huge files; for example, it's something I see when copying 60 gigabyte files. This knowledgebase article discusses the symptom for Windows 2000, though I've seen similar in Windows XP.)

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Hardware

Finally, we do at least have to mention the various pieces of hardware involved when you're copying files around. The disk controller, the hard disk itself, the system board and interface(s) to the disk controller and more. Typically, problems in these areas show up as something slightly more severe than just a slowdown, but since you indicate that it also appears to hang, it's something else that we need to put on the table for investigation.

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About the file size

I'm assuming that files significantly smaller than the 10 megabytes you mention copy without a problem. This is an interesting tidbit of information, but it doesn't necessarily pin down exactly what's happening. For example:

• Occasionally hardware related problems don't appear until some continuous threshold has been reached. For example, a disk controller might be having problems, but those problems might actually not manifest until it tries to transfer something over 10 megabytes (in your case).

• The process of creating a larger file might cause areas of the file system's directory structure to be accessed where smaller files might not. If the problem is with that area of the disk, it might not show for smaller copies.

• I mentioned the memory issues above where a copy program might use more memory to copy a larger file.

As you can see, it's difficult to say "here's your problem" because there are so many possibilities. Hopefully, one or more of the items I've mentioned will help resolve it.

I'd also like to ask readers to contribute additional ideas or things that they might have seen that result in the kind of behavior you're seeing. It's possible, for example, that combinations of software or specific software packages might have this result. (I wonder, for example, if perhaps some anti-malware software might be involved.) So please contribute and/or browse the comments to this article when they appear.