Hi, we are sorry to know this issue you have. According to the information you provide, we are afraid this is NOT a product quality issue with your Anker 4-port USB 3.0 hub.
This 4-port hub is one that comes without a power adapter, so it only has an output current that is about 0.9 amps to support devices plugged into it. That's why the hub works fine with your flash drives which are not high-consumption devices. But when the device to be plugged in is a USB 3.0 hard drive, this hub does not have sufficient output power to support a high-consumption device like this hard drive.
In this situation, you may need a more powerful hub, like a USB 3.0 hub that comes with a power supply so it should allow you to use the external hard drives with the powered hub.
Our support will reply you through email if you have any further questions. Thank you.