An idea is to have an offline two-way synchronization tool, where any of the backup copies can be modified independently. This is especially useful if you have several USB hard disks, and sometimes
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you add/remove/edit files on this drive, sometimes on that one...
From time to time, you connect two drives at the same time, synchronize the folders (directories) and continue with usual things. You now have the latest versions in both places! If you edited the same file in both places, you get both versions in both places - it is up to you to resolve the conflict. No data is lost!
What generally happened to me several times is to delete or rename a file/folder on drive A, but do not do it on drive B and C. Later on (3 months later) when I synchronize these files, I get duplicate files, or get again once deleted files.
In short, it helps you keep multiple copies organized.
Under the hood, git-sync uses an extremely stable and reliable Git version control system - the version control system created and used in Linux Kernel development. This means that the script only wraps the existing and very well tested git functinality for detecting and resolving conflicts. What comes for free (with git) is the ability to recover old versions of the files! :) And this means ANY previous version, practically instantly! [Less]