Voyager Modem


Hi I have a new Imac G5 i currently use a voyager105 usb adsl modem. When i want to shut down the computer the modem power stays on, as if there is no signal to turn it off. I have checked connections and all seem fine. After 1-2 minutes of me shutting down the computer the modems power seems to try and restart it or something and the fan or hard disc wer's like mad. Why is this please help Tony