I did this, NAS to NAS as storage density improved, with rsync checksums on. Years into this I noticed a thumbnail that didn't render, found I couldn't open the original. Looked around, there were quite a few.
Ran a RAW + JPEG error detection script: on the order of 10^1 - 10^2 of photos were no longer valid out of on the order of 10^5 - 10^6 photos.
Turns out rsync verifies on the way, not after write.
Propagating errors is indeed an issue. That's why, for example, my backup program never copies different files back from the backup. The backups form a tree, always writing down, never up.
Ran a RAW + JPEG error detection script: on the order of 10^1 - 10^2 of photos were no longer valid out of on the order of 10^5 - 10^6 photos.
Turns out rsync verifies on the way, not after write.