From your description, and only your description, you do not have an electronic connection to your USB device.
Confirm this by watching the console, using tail(1) with /var/log/messages, or xconsole(8).
Did you forget a power cord? Some external USB drives need external power, or 2 USB connections to draw sufficient power.
If it isn't power, then you may have a failiure of USB in the external drive, cabling, port, or internal to your workstation.
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