CB-C71 Cannot connect External HD & Phone

Hello!

I just received a CB-C71 hub. I have an apple 100W charger plugged into the PD port on the hub, and then a bus powered external HD plugged into one of the USB3 ports. If I connect an iPhone to the USB2 port, the external HD will unmount and mount repeatedly. Have I misunderstood the capability of this hub?

Thanks in advance for your time
K!

I suppose this is because the phone is trying to charge and sapping the power away from the HD.