This has been seen when connecting one or more PRE's to the Digidesign MIDI I/O or other USB powered midi interface with longer midi cable lengths (approx. over 25 feet). Typically, you will not need to use an external power supply when the MIDI I/O is connected to your computer, since the MIDI I/O is powered over USB from your computer. However, when using long midi cable runs or when connecting MIDI I/O to an unpowered USB hub Digidesign recommends using an optional external power supply to power the unit. Any standard 912 Volts DC, 500 mA; Polarity: Tip =Plus, Ring = Minus supply will work. If experiencing this error with a third party USB powered midi interface while connected to an unpowered USB hub, try a powered hub, or connecting the interface directly to the CPU.