Maxtor One Touch III Mini 160 GB showing only 4.5 GB space

I just purchased the above maxtor external drive; despite the software not working with Vista I intend to use the Windows backup function. My problem is the drive appears to have only 4.5 GB of space on it, not 160. It is recognized by my computer as 2 drives, F (with 2 GB space) and G (with 2.5 GB space). Does this make any sense at all to anyone? I am a relative novice so I’m hoping I am simply misunderstanding something.