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What could be the causes of variations in the speed of file transfer (copy&paste) of large data (multiple video files) from my computer to an external device?

I notice huge differences in speed, but this does not seem to be triggered by the use of other programs, like internet browsers. I am talking about one single such transfer (not multiple parallel transfers).

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I noticed the large transfer speed while Firefox was running, and then, without other applications, the speed may drop to about 8 MB per sec and lower.

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If there is a sudden increase in speed, followed by a gradual decreasing and another sudden increase, it is because there is a buffer somewhere in between the source and the target. The source fills the buffer, which is instantaneous (high speed), but then the source has to wait while the target is busy emptying the buffer as fast as it can. From the point of view of the source, this means that the speed decreases.

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  • The maximum speed seen in the image appears for a short time only. I have noticed that while copy/paste involves speed variations, cut/paste displays all the time a stable speed of about half the maximum speed displayed only for a short time during copy/paste process
    – user47206
    Jan 21, 2015 at 7:25
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    That is strange. As far as I know a cut/paste is a copy/paste followed by a delete, if the paste was successful.
    – Jos
    Jan 21, 2015 at 7:27
  • I made a mistake: in fact the speed variation appears in cut/paste too when transferring from computer to external drive. the stable 1/2 of maximum is for a transfer between two external drives
    – user47206
    Jan 21, 2015 at 7:58

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