Why does your computer not recognize your Western Digital passport essential hard drive? First you will need to connect your external hard drive to your computer. Then you will need to partition and format your external hard drive.
A how to install, partition, and format a WD drive on Windows Vista article from Western Digital support has some steps required. Click on a "Windows Vista" link to get correct instructions for Vista. Once properly partitioned and formated, you will be able to access your external hard drive.