This is something I've been wondering for a while now, and maybe (most likely ) somebody here knows the answer to this:

I'm using Windows 10 on a fairly recent quad core i5 CPU. This system is plenty fast. Which is why I've always wondered: how come doing two (or more) copy jobs at once slows the copy speeds down so much? I can understand it somewhat if you're copying to the same drive of the same computer, because that would mean the heads of the HD has to switch position all the time, but I've noticed this is also true for copying to devices external of the computer.

Eg. : I'm currently copying some folders containing mp3 files to my unRAID server on my LAN. At the same time, I would like to copy the same files to an external USB3 disk. Doing this at the same time slows copy speeds down to about half, meaning I make zero time gains by doing it this way. Ok, the HD's heads also have to change position because of this, but this is reading, not writing. And only about 20% of the CPU power is used, so that's not the problem as well.

Could somebody explain this to me?