When an error happens, the sync will try to continue until the end, processing as many files as possible. If one of the bases is disconnected, the sync will stop.
It is always recommended to have a look at each error. To view the error message, click the error badge that appears on the right side of the sync item, or select Show sync errors from the context menu. This will open the Activity log window, which contains all messages related to the file operations performed during the past syncs, as well as all errors. (The Activity log window can also be opened from the Window menu in the main menu bar, or from the status bar menu.) Selecting a log shows a more detailed message in the text area at the bottom of the window. Particularly for two-way sync items, it is strongly recommended to sync again by carefully going through the confirmation dialog and making sure that no unique file is going to be copied or removed because of a previous error.
System files
A common source of errors are hidden system directories like .DocumentRevisions-V100, .Spotlight-V100, .TemporaryItems, .Trashes or .fseventsd. These directories are usually located at the root of a device or in your home directory.
If the error happens on the source, it usually means that you’re trying to backup the entire filesystem (including system directories like /Library or /System which usually don’t hold personal files) or your entire home directory. SyncTime doesn’t have enough privileges to access system directories and is not designed to do a full volume backup; it is recommended that you only backup selected directories inside your home directory. If you still think that you need to backup directories containing one of these system items, you can either exclude hidden files or selectively exclude the system items.
If, on the other side, the error happens on the destination, it usually means that the destination is the root of an external device: in this case, in order to avoid these system-created files, you can create an empty directory on the root of the external device and use that as the destination.
Privacy settings
Another source of errors can be missing permissions for accessing privacy related files like your Photos library, Contacts, Calendar, Reminders and user directories like Desktop, Documents and Downloads. When accessing these locations for the first time, macOS shows a system alert asking for permission.
In case you denied any of them, you can enable them in the System Settings under Security and Privacy. To avoid syncing Contacts, Calendar and Reminders, you can exclude the Library directory.
Conflicts
One-way syncs with multiple sources and two-way syncs can produce a conflict error when, between syncs, a file is modified on more than one base, in which case it cannot be determined which file should be preserved.
A source of unexpected conflicts are .DS_Store and .localized files which are automatically created by macOS. Usually the best solution is to exclude them with a filter.
If you’re unsure why two files are in conflict, you can show them in the Finder by selecting the conflict error in the Activity log window and clicking on the file paths linked in the text area at the bottom. After selecting a file in the Finder, in the main menu select File > Get Info (or press Command-I). In the Info panel you can compare the file size and modification date, at least one of which should be different between the two conflicting files. To resolve the conflict, choose which file you want to keep and copy it over the other file, and run the sync again.