Some USB devices don't work when connected through a mixture of USB 2.0 hubs (high-speed) and USB 1.1 hubs (full- or low-speed).
An external hard drive may not mount, for example, or printing may stop in the middle of a job or not print at all.
If you experience this issue, change the configuration of the USB chain so that you are not mixing speeds. Connect a high-speed hub directly to the computer, and connect high-speed devices to it. Do the same for your full-speed/low-speed hub and devices.
If you have to chain hubs, be sure to chain only hubs of the same type. Note, for example, that USB keyboards with built-in hubs (such as the Apple Pro Keyboard) are full-speed/low-speed hubs. So for best performance the keyboard should be plugged directly into the computer or into another full-speed/low-speed hub.
To determine which type of hub you have, open System Profiler from the Applications folder. When you plug the device in, it will appear in the Devices & Volumes section with information about the speed of the device.