Two high-bandwidth devices or two isochronous devices cannot be connected to a single Universal Serial Bus (USB).
Symptom
When connected to the same bus, certain combinations of USB devices experience intermittent difficulties, or do not work at all. If one of the devices is disconnected or moved to another USB bus, everything works properly
Solution
Connect only one high bandwidth or isochronous USB device to each USB bus.
If your computer has a single USB bus, adding a USB interface card provides a second USB bus. Without a second USB bus, you may have to completely disconnect one high bandwidth or isochronous USB device in order to utilize another.
In general, two high bandwidth devices cannot share the same USB bus. Each USB bus has a maximum bandwidth of 12 Mbits/s. A USB device can request up to 6 Mbits/s of bandwidth. Depending on how they operate, high bandwidth devices may compete with each other and therefore may need to be placed on separate USB busses to work properly.
Isochronous devices cannot be shared on a single bus. USB supports isochronous devices, which can be high or low bandwidth. Examples of common isochronous devices include cameras and CD-R drives.