The maximum current that can be supplied by a USB port is 500mA at 5V. Some drives require in excess of this when spinning up. In this case they need to pick up power from two USB ports via a Y-cable.
Consult the specs for your product to ascertain its power requirements. Otherwise, if you know the model number of the drive inside the box, then consult its product manual.
Seagate Documentation Library:
http://www.seagate.com/ww/v/index.jsp?locale=en-US&name=sea-int&vgnextoid=059849b5e8ba0210VgnVCM1000001a48090aRCRD
Alternatively, check whether the following Microsoft utility can detect your drive:
http://www.users.on.net/~fzabkar/USB_IDs/UVCView.x86.exe
UVCView will also report the maximum current requirement of your device. Hopefully this will reflect the power requirements of the drive, but it may just report the current demand of the USB-SATA bridge chip inside the box.
