I was just diagnosing a sleep problem that turned out to be caused by a malfunctioning iDsonix USB3 hub, but in the process I noticed an unrelated weird behavior, and I'm wondering if it's the hub, if there's actually something wrong with my computer (5K iMac), or if this is expected OS X behavior. If you've got a USB3 hub and at least one USB2 and USB3 device handy, it'd be easy to test. The behavior I'm seeing: If I plug the USB3 hub into a port on the computer, then immediately connect a USB2 device to the hub, it works fine (at USB2 speeds, of course). However, subsequently, no USB3 devices, at all, will work on that port until it's been disconnected. Not just the hub, but the port on the back of the computer itself--if you plug a USB3 device into the hub, or swap the cable from the hub to another USB3 device, the computer completely ignores it. Unplugging the USB plug from the back of the computer, however, and re-connecting it to the same port, appears to reset that port and cause it to again function normally. If, however, I first plug a USB3 device into the USB3 hub, it shows up as a USB3 hub and behaves normally. If I subsequently add USB2 devices, they appear in the device tree as a being connected to a separate USB2 hub, but the USB3 hub doesn't disappear, even if there are no USB3 devices connected, so the hub acts normally. Basically, this hub is acting like its USB controller is "asleep" and doesn't show up to the computer at all until at least one device is connected. When a device is connected, it will register USB2 or USB3 hubs as necessary to support the device(s) connected to it. But if that first device is a USB2 device, the only hub the computer will see is a USB2 hub, and the computer will subsequently disable the USB3 pins on that port, although the connected device thinks it's a USB3 port. Disconnecting the cable from the back of the computer appears to reset the port, but disconnecting the B end from the device does not. I could imagine this being an OS/firmware issue. I could also imagine this being a weird glitch that results from an OS/firmware peculiarity that's the result of this particular hub's chipset doing unusual things in order to save power or something. What's the behavior you're seeing? This, or does it behave like you'd expect? Note, again, that once the port has had a USB3 device connected to it once, it will behave normally until you physically disconnect the USB cable from the back of the computer--even connecting the device at the other end to swap to another device won't do it. So it'd be easy to test a USB3 device once, leave it plugged in, and never notice subsequently.