I connect with a 3.0 USB port.
Did you format your hard drives or use them out of the box as is?
USB 3.0 is still pretty slow copying that much data. Try robocopy it’s much quicker. You can also use Veeam (it’s free) which would allow you to backup one hard drive into a disk image and then you clone the other drive with it which is pretty quick if the drive is an ssd.
Double check the cables you are using. Many USB-C cables are intended for charging, and are limited to USB 2.0 data transfer rates.