Differential backup of a single file I have a single file, a database for Photos, that I regularly backup to an external, however instead of resaving the entire file is there a program that will only differentially resave the difference?

I'm on a Mac Pro 2011, running Yosemite 10.10.5.
A Photos library is not a single file, it's a package. However, the problem with differential backups of packages is that the contents relies on other parts of the contents. You can't restore part of a package—it'll break the entire package. The solution to this is just to replace the previous package backup on backup.