Click a button, and the program copies all files that are missing in one folder from the other folder. Both folders can have the same name, which suits the setup I use backing up to Zip disks. What I have been doing is checking the latest date on the destination folder and copying everything created after that date from the master folder on my hard drive. Not foolproof, but it works passably. However, FoldersSynchronizer 1.7 does the same thing a whole lot better.
New files are copied in both directions if that applies.
The user can save and load the lists. Two AppleEvents applet samples are included with the program. This is a very slick little application, with an attractive interface. I think my backup quandary is solved. For more information or to download FoldersSynchronizer 1.7, visit the FoldersSynchronizer page.