I have two USB devices attached to my mac. The memory runs at about 65.7MBps no matter what i do. Ranges from about 64.0-65.9. USB doesnt make a difference. This measurement isnt of how much my total system RAM can move per second- i have 320MB- i think its how much - well you know. . i think. Anyways, i have USB ports because i installed a PCI card- which has some sort of dedicated processor on it. My guess is that you use an integrated USB channel soldered to the board, w/o an I/O controller, so your system controller has to send instructions, using extra CPU cycles and comprimising your RAM because it has to do more than it would've otherwise (the ram i mean). If you have a USB PCI card floating around for some reason try installing that and see what happens. Also if you have two separate USB ... . channels i guess(?) try connecting one USB device to one and the other to the other, or if you're doing that now try plugging both into one.