Mac Pro USB ports competing for power?
The iPad is a high power draw device, and Belkin N150 USB Wireless adapter probably is no power slouch either, so it is technically possible for them to be overdrawing an internal USB Bus or Hub in the Mac Pro.

The Mac Pro has 5 independent USB Buses as noted in the picture below.



Now depending where things are plugged in they show up in different USB Buses, you may want to Open your System Information Application, and try to avoid placing the iPad and the Belkin N150 on the same bus.

For Example, on my computer the ZyXel... and the DataTraveler are plugged into front USB ports and they appear to be sharing the same usb tree. While my external Hub labeled "Generic USB Hub" and my VF0680, CIRCA102 are all plugged in rear usb ports on the Mac Pro, note how the CRCA102 is on its own Bus.