I am using the passport as a storage device only. I have copied many documents and backed up my system. I don't know why my hard drive only has 45 GB used but the passport showed 190 GB saved on it. Is it not just updating the data? I would imagine that if it sees the same thing it isn't going to copy it as new data, just updated (as in changes)
Check the percentage of space used by the recycle bin on the external hdd, and whether system restore is turned on for the drive, both of these could conceivably use up a good deal of space. If you're using Windows Vista/7 is your system using the external drive for doing system backups?