First check that it doesn't just need a drive-letter manually assigning to it:
Control Panel > Admin Tools > Computer Management > Disk Management
In the bottom-right panel, find the ribbon which relates to the Passport drive (No name & "RAW" file system).
Right-click and choose Change Drive Letter & Paths.
Choose a letter.
If you need to replace it, I recommend you buy either a Western Digital or Hitachi. I've had several Seagate external drives fail on me in the past 18 months. The other two brands are still working fine.