Smart Sync failures: online-only files re-downloading and reverting

Smart Sync (introduced in 2017) lets users mark files and folders as 'online-only' so they occupy almost no local disk space while still appearing in the file manager, downloading on demand. The feature had been previewed under the name 'Project Infinite' before reaching general availability for Business and Plus users in 2017. Its reliability promise is that the local/online state a user sets will stick. In practice, users have reported for years on Dropbox's own community forums and Apple's support boards that online-only files keep re-downloading on their own, sometimes pulling entire folders back to local storage and unexpectedly consuming large amounts of disk space, then needing to be set back to online-only again.

A recurring cause is that any process which merely reads files — backup tools, antivirus, indexing, 'recently used' lists, or third-party utilities — can make Dropbox treat the file as accessed and pull it back local. In at least one documented case a popular Mac cleanup utility's interaction with Dropbox was acknowledged in Dropbox's knowledge base and reportedly fixed, only to resurface. Users have also reported the Dropbox cache failing to clear automatically, leaving tens of gigabytes of supposedly deleted data lingering on disk.

These are not catastrophic data-destruction events like the 2014 Selective Sync bug, but they are a sustained reliability problem: a feature whose entire purpose is to manage local storage repeatedly does the opposite, silently and without the user's intent.