Had similar problem with my Passport 350gb
read this and noticed someone posted Western Digitals recommendations.
"Cause:
There may not be enough power currently supplied to the machine's USB port. Most computer systems only supply 500mA of power through USB ports. The WD Passport hard drive requires a little over 1000mA of power at spin up."
Turns out I used a different USB cable second time I plugged it in. Went and got my original cable and worked fine. Other cable was obviously thinner and not as powerful. Good to note, hope others learn from this too.
