Quit all unneeded applications.
Disable system preferences or "auto-loading" utilities that "probe" the hard drive or USB bus.
Many USB 2.0 hubs disrrupt the bus, even when not being used by the XLR8 devices.
Examples of these are Stuffit AVR, Acrobat Distiller "watch" settings, Energy Saver, Auto-time settings, and sometimes, AppleTalk and/or sharing services.