Question Slowing external drive

I got an external HDD and at first it writes decently at 100Mbps and it did so for much of the file transfer; but then it begins to slow down and the drop in performance is very sudden it goes down to the 10s of Kbps or lower and rarely bumps back up to the 5-10 Mbps (when it does its usually for just a few seconds). Usage has remained consistant with the external drive showing 95%-100% at all times.

I am transferring files from a WD Blue (5400 rpm [i think]) onto a Seagate 2tb external drive. Using USB 3.0 port on my Motherboard and SATA III interface on the WD Blue.

Can anyone help me figure out why the drop-off is so fast in the file transfer since it did not seem like a typical disk drive drop-off?