I used a paid SaaS sync service 10 years ago (not Dropbox) that had the following failure mode even though it had been around for a few years..

You could have it mirror an entire subdirectory, including external drives.

If you booted up long enough and that external drive was not mounted, the service registered that as a subdirectory delete (bad). When you then mounted it again, the sync agent saw it as out of sync with the newer server-side delete and proceeded to clear the local external drives.

They also implemented versioning so poorly that a deleted directory was not versioned, only the files within it. So you could recover raw files without the directory structure back in a giant bundle of 1000s of files. Horrible.

See: https://dynamicsgpland.blogspot.com/2011/11/one-significant-...