Bus-powered describes a USB device or hub that gets all the electricity it needs directly from the USB port it is connected to, without a separate power adapter. Many small accessories, such as flash drives, basic mice and simple hubs, are bus-powered for convenience.
The amount of current a USB port can supply is limited, and this limit is shared among any devices connected through a bus-powered hub, since the hub itself has no extra source of power to add. This is why plugging several power hungry devices into a bus-powered hub can lead to devices behaving unreliably or not being recognised at all.
For accessories that need more current than a single port comfortably provides, such as external hard drives or hubs feeding several devices at once, a self-powered setup with its own adapter is the more reliable choice. See our guide to hubs and docks for practical recommendations.
