Question why does the transfer speed vary so much?

[brannsiu]

This is not a perfectly new PC, but this is a very clean PC, the windows was just cleanly reinstalled
and there is only one or two trusted third party programs installed.

I was trying to cut and paste 100GB video files which comprises 111 files from the PC to a
portable passport USB 3.0 western digit, the portable had drive is perfectly new, and just unboxed.

Speed of transfer varies from 30MB per sec to 100MB per sec and it's often not steadily. It rises up
sometimes and drops down sometimes, the range is quite large.

Nothing consuming is doing at the background. Just visiting the forum of Tomshardware now.

What is the reason? Is there anything I should do when transferring large files so that I can reach the a higher transferring speed?

 

[mdd1963]

Normal for speeds to vary....; the drive will seem fastest for short writes on files that can fit within it's typical 32 MB cache, so it can cache it very quickly, then write it gradually.

It's not as if a spinning drive is going to benefit 'that much* connected to USB 3.0 when transferring large files, and, sometimes, USB 3.0 circuitry within some external drives can throttle or heat up on very large transfers, either writes or reads. (Not saying that applies to your WD drive, but, I've heard of it happening with some external USB 3.0 drives)

Test it with a USB 2.0 port and compare...

 

[jsmithepa]

I would run a diagnostic on the source drive just in case.

 

[brannsiu]

I would run a diagnostic on the source drive just in case.

So did you mean what I was experiencing is abnormal??

 

[popatim]

No, it's normal. Your passport is no speed demon. Even if it was the larger external HDD, you would still see something similar.
I'm assuming this is for your peace of mind.