Verify the power supply at the external unit and be sure that the power is being supplied to the device. Most external media devices have power lights to indicate whether the power is present or not.
Next check the removable media, if present.
You should check on the Device Manager to ensure that the device has been recognized and also to check for conflicting device driver information incase the Device Manager is not able to see the device after the correct driver is loaded. The final step would be to check the signal cable by replacing with a known good one.
