USB devices may not work correctly when they're connected to certain USB ports, e.g. ports in a hub or on the front of a desktop case.
See response 3 in this:
External drives require a lot of power, and they won't work properly in all possible USB ports you can plug them into.
See response 1 in this:
You can buy a PCMCIA (PC Card) USB 2.0 adapter, or an ExpressCard USB 2.0 adapter for your laptop if you need more USB ports. Those are more likely to work with more USB devices that a powered hub.