My USB printer does not work

Cause: USB hubs allow a number of different devices to connect to them, but some devices can pull too much power, causing other devices not to work or not to work well.

My USB scanner does not work

Cause:  USB works with hubs, attachment ports to which you can attach USB devices. A USB hub can either be self-powered or bus-powered. A self-powered hub has a wall outlet that you plug in, in order to power the devices attached to the hub. Bus-powered hubs get their juice from the computer. Some devices, such as scanners and portable hard disks, need more power than the USB bus-powered hub can provide.