First I suggest that you use a more precise timer to reduce the chance that the effects you are seeing are the result of the way you are measuring rather than differing performance of different hardware. Look at keQueryPerformanceCounter (https://msdn.microsoft.com/en-us/library/windows/hardware/ff553053(v=vs.85).aspx)
If you still see a large difference, then it will be time to checkout cacheing and other effects
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Tim, Alex, thank you.
to alex, I will post the code later.
to Tim, I know the exactly expression need more parentheses.
the test result of write speed using my program looks OK, it usually would be 15MBs ~ 100MBs depend on different USB 3.0 disk.
but the test result of read speed was very tricky, it would be very different on different PC, sometimes may be 700MBs ~ 1500MBs, sometimes only 2~4MBs, the gap is too large, the normal value of read speed should be 100MBs~250MBs depend on the quality of USB 3.0 disk.
I wonder if it is the IO buffering issue.
Thanks.
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