Oh dear.... Just culled this from the Iomega website. I already use one of their external discs, but in the light of this thread, wandered off to do a bit of research for you. It does not mean that this applies to your case, but it is something else to consider.
From the Min PC Specs...
• 300 MHz or higher with certified USB controller (some USB controllers or motherboards may not support all types of USB devices).
Conan