This kind of problem occurs sometimes with USB-powered external HDDs which use a "power assist" or "power sharing" cable for drawing additional power from a second USB port. It's worth noting that bandwidth is also shared between ports sharing the same root port hub.
In any case, the available/consumed bus power can be confirmed by examining the properties for each USB root port hub in Device Manager.
On many PCs/laptops, each USB port connector will be connected to a seperate root port hub and so be able to supply full power, but there are some where ports are shared.
Using a powered USB hub can avoid this concern, but can cause initialization issues with some USB devices which need to be connected directly to a root port (issues can also arise when using a USB port provided on a laptop's docking station).