Why does my External Hard Drive show more GB than it's supposed to?

Hi,

I have an Acomdata 150 GB External HD. I am using it to back up my home folder* which is 30 GB. I have done this back up about 8 times now since I got the unit. I will drag these folders onto the Ext HD icon. Once copied, I delete the previously saved home folder. (When both the old and the new home folders are on the Ext HD, that's about 60 GB.) Now, Ext HD Info tells me that the capacity is 148.93 GB, and the available is 29.99 GB. Info says that 'Used' is 118.93 GB on disk. I've double checked the size of my home folder and it is @30 GB. The previous home folder, that I will trash, is also about 30 GB. I ran the disk utility on the Ext HD to verify disk and it said everything is OK. What is going on?

Thanks, Rafael

*Desktop
Documents
Library
Microsoft User Data 435MB
Movies
mp3s Audio
Music
Pictures
Public