How long is it taking?
I suggest you read this article (even though several years old, what it says still holds): https://www.pcworld.com/article/2360306/usb-3-0-speed-real-and-imagined.html
When you combine the fact that copy actions typically get low priority if the OS has anything else to do, you're dealing with a platter drive, and you're copying 300GB of data that means it's going to take a while. I'd be shocked if you could get this done in under 30 minutes, even if it's all you're doing, and it will probably take significantly longer than that.
The time estimates shown for the copy command for this much data should never be trusted and virtually never take as long as the estimate. The larger the amount of data the shakier the estimate. The actual transfer speed during a long copy such as this will zig-zag all over the place depending on what the hard drives are doing and what the OS needs to be doing besides the copy, which is always quite a bit.
If you're actually trying to copy 3 TB this will take a very long time, hours and hours at least.
Edited by britechguy, 26 May 2018 - 09:42 AM.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story