Thats normal for that drive. It's got a small fast buffer but after that it drops to it's slow native speed and cycles up and down like that as the buffer fills & clears, over and over. It's like an old slow usb2 drive someone slapped a usb3 interface onto to get people to buy it.
copy from the main hard drive to the USB drive on USB 2.0 Port , the issue here not max speed as the max speed in my case is Ok for USB 2.0 but the issue is the sin wave performance is down to 300KB
i have tested different ports on PC.
i also see the same effect when connected it to USB 3.1 Gen 1 port on laptop the sin wave is still there but the min and max speed is higher because of USB 3.1.
the main reason i have asked to know is this normal for all devices with same model and prand
o i just got a defected one.