Suppose that we have two replicas A and B of a large data set in locations that don't have a good Internet connection. As a rule of thumb, let's say it's possible to transfer metadata but not actual file data, but a full offline solution would be even better. One of the replicas is a master, the other one is sort of backup that needs to be updated semi regularly.
The way the two data sets are kept in sync is... old school. Someone copies the files that need to be updated to an external drive, physically gets from A to B where the files are updated in the other replica. However, it gets increasingly difficult to keep track of which files need to be transferred, especially in error cases like a corrupted file.
It's impractical to have a full copy of the data set on the external drive as it's at least an order of magnitude larger than its capacity. A typical file size is a few gigabytes and the entire data set shouldn't grow past lower tens of terabytes in foreseeable future. A typical update is a file addition, updates to existing files and deletes are fairly rare.
What matters the most is the bytes being preserved, I don't mind about inconsistent timestamps or file permissions.
One can do this manually storing checksums of files in replica B on the portable drive and checking for mismatches on replica A. But hey, is there an established tool that supports use cases close enough to mine?