I ran out of ports with iMac so I went and bought a USB hub thinking that I can plug my ipad mini into that and then have the hub plug into my computer. It didn't work. My first thought is maybe the hub isn't powered so I went and bought another usb hub this time a powered usb hub. Came home and tried it, same problem. The top of the mac mini status bar says "Not charging". What's going on? Is there a specific type of hub I need to get?