I have a 2008 Mac Pro, and on the usb bus I have a Duet 2, Logitech Camera, Mouse, Keyboard, Apple Cinema Display and Western Digital My Passport connected. Sometimes when I'm using the logitech camera, the image may get dark, like the camera cannot create enough light in the image, and a plug off and back in works to fix it. Sometimes Duet 2 will shut down and again a plug off and back in works. So I'm thinking, the USB bus is not getting enough power to support all the power draw of these tools. Does anyone know what's the limit for Mac Pro's USB bus power?