Everything has to be USB3.0. You should at least read disk A at 100 MB/s and write to disk B at 70+ MB/s simultaneously. Use rsync which shows current speed 
rsync -a --info=progress2 src/ dest/
I’ve not tested dummy files created by dd, but they should as well trigger this error. So test different dummy files from 1 GB to 50 GB.
I’ll do some more testing as soon @balbes150 has included the XHCI patch into his next Armbian build 
If the patch does not solve this I’ll try the USB2 port on my VIM3 to see if the USB2 interface is affected as well. I’ve not tested USB2 yet as it should be fine. Time will tell.
Update:
@Frank, if you get the same error as I did, you need to try the patch dwc3/core: xHCI host not responding to stop endpoint command which solved the issue for me as I’m now able to copy/move files between two drives over USB3.0 interface simultaneously. The patch should be added to the vendor kernel Khadas use for VIMs to prevent others from facing the same issue. The patch will eventually be merged upstream into mainline if either Amlogic or Hardkernel follow it up. More info at Hardkernels forum.