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I have a large directory, which I occasionally want to backup to my NAS and to an external hard drive. I work a lot in this folder, but the majority of data (all my photos and videos) does not change, so deleting the backups and writing the entire folder again seems inefficient and unnecessary. Is there an easy way to update/one-way synchronize these backups and make sure they're exactly the same, i.e. add new files, delete files not present in the original folder anymore, update files with the same name but different checksum, and leave everything else as is? Can Windows 10 do this already on its own? Small command line utilities are fine, but I'd prefer not to install larger applications for this. If such a tool exists, I'd prefer if it was also available for macOS and Linux.

My apologies if this is a common question, but I could not find any solutions to this specific use case.

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Windows 10 contains the command-line tool robocopy.exe which is a perfect match for copying data sets to external drives e.g. for making a backup.

Especially the parameter /MIR is useful as it allows to mirror a whole directory tree from the source to the destination. Existing files that have the same modification time and size will not be copied again, new and modified field will be copied/overwritten. Directories and files that exist only in the destination (may be because they have been deleted in the source directory) will be deleted.

My personal favorite robocopy parameter combination for making a backup is this:

robocopy <source> <destination> /MIR /XJ /NDL /NP /TEE /LOG:logfile.txt
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    This would benefit from adding /Mon:1 and adding it as a task in Task Scheduler to automate the entire process.
    – JW0914
    Commented Jan 18, 2020 at 12:40

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