SD wants to erase my partition
 

Downloaded the trial version. When I tried out a full backup. The destination was a partition named "backup" on my external hard drive. It said it would erase "backup" then backup the files. (i'm assuming it meant it would erase the files in backup) Thats not what it says on the screen in your advertisement.
I have other files on the backup partition I want to keep.

I have a
White 13 Macbook Core Duo
10.4.9
60g HD half full
2Gig ram

External drive is 3-90 gig partions

It depends what copy method you use: see the User's Guide -- specifically "Storing a backup alongside other files on a destination drive".