Windows has probably failed to assign a drive-letter to your Passport drive, which means Windows Explorer cannot show it. You can fix the problem by manually assigning a drive-letter to it:
Control Panel > Admin Tools > Computer Management > Disk Management
In the bottom-right pane, find the ribbon which represents the Passport drive (it will have no name or drive-letter, and may have "Raw" as the partition-type).
Right-click it and choose "Change Drive Letter & Paths", then click "New".
Choose a letter from the drop-down.
Click 'OK' or 'Next'.