This may be old news, but the PowerBooks do not supply enough power to use the WD external although my LaCie drive has always worked fine.
Here is what WD customer support wrote to me. This is NOWHERE on their support pages, so I think they wasted my time and money. An powered hub can solve the problem.
Thank you for contacting Western Digital Customer Service and Support.
The Apple PowerBooks have been known to not supply enough power to its USB ports to power some devices. You will need a USB 2.0 Power Booster Cable to run this drive on your system.
USB 2.0 Power Booster Cable
WDCA029RNN
http://www.wdc.com/en/products/accessories.asp?ProdID=170
Disk does not mount. I am using a 15" G4 powerbook. I think the most likely answer is that the USB port does not supply enough power.