Packages are folders that may contain a number of different files and folders inside, but are meant to work as a single atomic unit, and appear as a regular file in the Finder. The internal structure of many types of package are private, and changing one part of the package independently from another part can result in an overall corrupt package, even if those changes technically occur in separate files in the package.

As such, Synk also treats packages atomically, like they were single files. The resulting full package structure and content from a synchronization will entirely match one of the input packages, and changes to different parts of the package in the different copies will be considered a conflict. Old packages will be archived in their entirety when a portion of it changes, so the archive contains a self-consistent copy of the data.

Starting with iPhoto ’08, iPhoto now stores its library of photographs as a package. However, when you’re backing up your iPhoto library, you probably don’t want Synk to have to copy all the files any time any single photo has been touched or added. You only want Synk to copy the changes. Therefore, for some packages, Synk ignores the fact that they're packages:

• Packages whose name contains “iPhoto Library” somewhere in it verbatim (iPhoto libraries)
• Packages with extension .aplibrary (Aperture libraries)
• Packages with extension .approject (Aperture projects)
• Packages with extension .lrdata (Adobe Lightroom libraries)
• Packages with extension .dvdproj (iDVD Project)
• Packages with extension .iMovieProject (iMovie Project)
• Packages with extension .sites2 (iWeb file)
• Packages with extension .vmwarevm (VMware Fusion VMs)

Therefore, if you want your iPhoto library to only copy the files that have changed, make sure that “iPhoto Library” is somewhere in its name. For example, “Summer vacation iPhoto Library”, “iPhoto Library for 2009”, and “The best iPhoto Library ever” would all be handled incrementally.