[parasubvert]

Samba (on Linux, not just Mac) has always had corruption issues for large files related to locks and filesystem caching quirks. Windows and the protocol itself has a different approach to locks than *NIX. Many have gotten more stable by disabling caching (though this hurts performance on low bandwidth / high latency fileshares).

Also from what I recall, Apple SMB is based on a forked version of Samba from 10 years ago. They did this when Samba went GPLv3. Not clear how well they’re keeping up with quality improvements.

My solution: use WebDAV on your Synology for transferring files from a Mac or Linux host. It has its own quirks but is far more reliable. Unfortunately this won’t work for time machine. But would work with “rsync -aXv —partial”.