BMM389EC
Laptops are notorious for not providing enough power to USB ports, especially if you are running off internal battery (I had an Acer that was very bad at this). Lots of USB devices tend to drain the max current, and the poor little laptop logic decides that there's been a power failure.
Somewhere (you don't say which Operating System you're running) there is an option on USB to ignore power errors and carry on, but that will only diable the USB and continue booting.
Best solution is to try and run off mains power whenever possible, and buy a powered USB hub (Belkin - available from PC World off teh shelf, or from
www.ebuyer.com if you want to save money), and let it provide the power to the USB. That way you aren't risking any power issues either.
Good luck