USB powered drives aren't supported for Dish receivers. The power supplied by a USB port (0.5 amps @ 5 volts) is marginal for some drives. USB 3 ports do officially support more power (0.9 amps @ 5 volts) so you may be able to power drives that are marginal on a USB 2 port.
It's also possible Hopper does not supply as much power to the USB ports as the USB spec demands. This could explain why Dish limits the devices it supports via USB.
A workaround MIGHT be to insert a powered USB hub between Hopper and the drive BUT that doesn't get around the issue that some drives draw more power than a USB port is designed to support. The hub is allowed to shut down a port if a device draws more power that allowed by the spec.
Best advice is to use a powered external drive as recommended by Dish.