This issue has occurred few times on my Windows 10 PC running Synology Cloud Station Drive. The Cloud Station software would get stuck syncing one or more seemingly random files. Status of the file would be stuck on “Preparing” with progress bar moving back and forth, but never progressing past “Preparing” stage.

Deleting the file(s) would put the drive seemingly back in sync, but trying to copy the file back would cause the same issue. One thing I noticed was that when the file was deleted from the affected device, other devices syncing the same folder (including File Station on the NAS itself) would still show the file in its place and all devices were displaying that folder was in sync! In a similar way, if the file was renamed on the affected device, other devices (and File Station) would show both files (original and renamed), but the affected PC would only have the renamed version. Again, Cloud Station on all devices would think that everything was in sync.

The work around was to delete the affected file from File Station web interface (make a backup first). After this, the affected PC would immediately upload the file back and Cloud Station would then sync it across all other drives.

This is obviously pretty serious issue as it leaves synced folders in inconstant state while displaying that everything is is sync. If this happens again, I will raise the issue with Synology.

August 2016
Synology Cloud Station 4.1
Windows 10 Pro