While all USB ports provide some amount of power for attached devices, the available power may not be enough for certain high-current devices such as USB hubs or external hard drives. High-current devices usually come with their own power adapter, making them self-powered, in contrast to a bus-powered device that draws all of its power from the host computer's USB interface. Bus-powered devices can cause issues if they need more power than is available from the host machine.
Many of our devices that include power adapters, especially USB hubs, will function in either self-powered or bus-powered mode. However, even though the device may function, each additional device attached to the host computer reduces the total available bus power. If the power runs out, any USB device attached to the computer may suddenly disconnect. If this were to happen to a USB storage device, such an event could result in permanent data loss.
If a device comes with a power adapter, we recommend that the adapter stay connected at all times, otherwise the device may not function as designed.
Self-powered USB device - A device that takes all of its power from an external power supply
Bus-powered USB device - A device that takes all of its power from the host computer's USB interface.