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I've been meaning to ask this question for a long time. While trying to copy/move files(large sizes say over 10GB) in Windows from or to any external drive, I notice that the transfer activity remains homogeneous until 99% where it stops responding for a minutes and then completes the few remaining bytes.

I don't understand the logic behind this as I've noticed it only in Windows and not in Ubuntu. Both internal and external drives are SSDs. It happens with flash drives too. The CPU is a ULV processor, could it be the problem?

Thanks!

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  • 1
    Because the % estimate in Windows is not accurate most of the time.
    – Moab
    Mar 12, 2016 at 3:59
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    Try disabling your antivirus/antimalware to see if it helps? Maybe the antivirus is checking the files so that's why it is taking too long to finish.
    – axys93
    Mar 12, 2016 at 4:06
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    Also it is a common issue when transferring files from a fast source to a more slower destination. If the external drive is connected via USB, it can diminish the real transfer speed. If the file is very large, the speed goes slowly down to the real transfer speed between the two devices, because new data only can be read in same amount as the old data are removed from buffer. So when reaching the end of the file, it 'pauses' for a moment waiting for the data in buffer be transferred to the target. So the only solution in this case is wait windows transfer the cache content.
    – axys93
    Mar 12, 2016 at 4:17
  • @axys93 have tried disabling the antivirus but still the same. about your second comment..yeah it might be the reason
    – Ashwin
    Mar 12, 2016 at 4:34
  • I think Windows Update has a similar issue - the bigger the update, the longer it sits at "100% downloaded". Check net traffic and see that it's still busy downloading for minutes more after "100%" so it is not really at 100% - same with large file transfers. The algorithm used to display percentage completion is over-eager to say these things are done when they are in fact, not.
    – rdtsc
    May 8, 2023 at 13:34

2 Answers 2

Reset to default
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Caching.

(Probably)

When Windows writes to disk it caches the writes to the disk. It shouldn't be doing this for flash or external USB drives, but it's the only real explanation that fits.

The copy file dialogue is showing the speed to copy files at the cache and not the disk itself and is then waiting at the end for all the data to be written out. The copy program can presumably see that there is data in the cache waiting to be written out but not how much.

There is a way to disable the write-cache for drives which may well stop this behaviour.

There is a quick guide to disabling the cache on the disk at Seven Forums, but I'm not at my computer to summarise it properly here. Basically you want to disable the disk cache only on removable or external devices, for performance you want to leave it enabled on internal devices.

You should be able to get to the same properties pages shown in the tutorial I linked by right-clicking the disk in an Explorer window, going down to properties, then selecting the Hardware tab that shows at the top of the window.

It should be a bit like this image from The Windows Club but you should have an option for quick removal on USB devices. (I'll replace this when I am able to later)

enter image description here

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    I just experienced this problem while copying data (> 500GB) from D: to E:. Though your explanation sounds reasonable, the ram appears to be almost completely empty and there is no activity on disk C:. On the other hand, both the disks D: and E: appear to be active (the former is read, the latter is written) at full speed three hours later the 99% figure has been reached. So I am not sure cache or buffering are the real general answer to this windows issue.
    – Patrick Trentin
    Jun 20, 2017 at 21:41
  • This does not only happen for large file transfers. It also happens when deleting files. And 13000 files are not that much...
    – Thomas Weller
    Mar 17, 2019 at 11:14
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    Definitely not caching, because I move around 1000 folders containing roughly 100k (200GB) files from/to folders in the same drive and the transfer window stucked at 99% with remaining 0 bytes for some good minutes. Not sure if windows was updating some kind of table or it's gathering metadata information for telemetry, or else. I didn't see any of my drives doing heavy work while at that.
    – Skyvory
    Jul 17, 2020 at 9:00
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    For me the culprit is Windows Defender. If I disable it before doing large amounts of deletes / copies or drag and drop extracting of archives the issue disappears. I reported it through feedback in Windows. It was especially troubling since I have a PCI-E 4.0 NVMe SSD with 32GB RAM on a 5950X. You should never have to wait that long for something to complete with that system.
    – showp1984
    Dec 6, 2020 at 17:03
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For me it was an old sata cable! i gave it a jiggle at the connection to the port on the mobo and it miraculously worked again with no issue.

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