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I have an external hard drive that I needed to copy about 30 GB from. 8 DVD image files. The copy started out at 160 MB/s, quickly went down to around 80 MB/s and then slowly during the process it has now gone down to 11 MB/s (2.7 GB left)... What is going on here? Why does this happen?

Using Windows 7, copying from an NTFS formatted USB drive, SAMSUNG HM160HI USB Device. As far as I know it is connected to a USB 2.0 (otherwise I think Windows would have complained?) and it is platters, not solid state.

I'm getting the transfer speeds from the regular Win7 copying dialog (after clicking the Show Details button)

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Is this a USB drive? Is it in a USB 2.0 port? Is it solid state or platters? Where are you getting the transfer speeds from? – JNK Sep 10 '10 at 13:45
I've recently been moving large amounts of data around and, upon investigation, found this effect everywhere. It had me completely stumped. – Zaz Oct 26 '10 at 22:14
There is a later question…, so the other would be a dupe (but it may be worth checking the answer). – sancho.s Apr 30 '15 at 11:15
up vote 10 down vote accepted

OS and drive buffers have filled up/been exhausted, and now you're transfering data as fast as the hard drive can supply/consume it.

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+1 - It's also possible the initial estimates were inaccurate, or that there was compression coming in to play that's not applicable on the whole drive. – JNK Sep 10 '10 at 13:58
Is there a program or something I can use to measure where the bottle neck is? Not extremely important, but would just be interesting to see if it's my system or the drive or the connection or what it is. – Svish Sep 10 '10 at 14:08
The bottleneck on any modern system is almost always the hard drives. Poor-quality controllers may also contribute to it, but this is not testable without removing the drive from the enclosure. – Ignacio Vazquez-Abrams Sep 10 '10 at 14:13
Disk IO is your limiting factor 9 times out of 10, especially with physical media (i.e. not SSDs). I work with very large databases and it's our bottleneck too. – JNK Sep 10 '10 at 14:14
@Savish: The system as a whole can only go as fast as the slowest part. A USB cable is usually slower than a hard drive, so will limit data transfer speed. – Zaz Nov 2 '10 at 18:26

I don't know why this happens but I have the solution:

I was also having the same problem. I checked that my CPU usage during copying was not any more than 5%. I refreshed on the dektop so the CPU usage increased and the copying speed also increased.

So what you have to do is press and hold the F5 key during the copying. I'm sure it will help.

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