Wd mybook 320 gb external hard drive will not shut off even when the power onoff button is held down and MacBook Pro laptop won't recognize it anymore after 2 yrs of no problems until now please help?

You should try re-installing the software that came with your external hard drive. Or, you can simply turn off your har drive manually after your shut down your Mac. If you are worried that you may cause data damage if you disconnect your external hard drive from your laptop before it stops processing, as long as your MacBook Pro is completely shut down and turned off, no data is transferring between that and your hard drive so it should be safe to manually disconnect it.