USB 2.0 10 port hub prevents mac from booting

I was using the USB 2.0 10 port hub with my iMac running OS 10.8.4 and it worked for a couple weeks. Then, it was preventing my computer from booting up on restart. The only thing that would allow my computer to restart would be unplugging the hub. I’ve tried plugging into different ports, but the same problem keeps happening. Any ideas on how to fix this?

This issue was resolved by unplugging an external hard drive that was plugged into the hub.