It is not possible to automatically synchronize a group structure between databases – and since tags are groups, this limitation applies to tags as well.
You have a few options:
- Manually manage the problem. Create a tag in each database where you want that tag. Not much fun, but not a huge burden usually.
- Select several tags in one database and use “Duplicate” to copy them to another. Big downside here is that any document that has been tagged with those tags is also copied – making the post-copy cleanup a burden and much more work than (1), above.
- Maintain the tag structure outside DEVONthink. You could make a folder on the desktop or somewhere and make subfolders for each of the “tags” you’ll be using. Then, import those folders into the “Tags” parent group and you’ll have new tags with those names. This soon all becomes more work than is worthwhile, and that takes me (personally) back to option (1).
- Keep your tag hierarchy in the Global Inbox (or some other database) and do all your tagging there before moving to another database. Moved or copied documents retain the tags.
Never happens over here – maybe there was operator error? If you have enabled groups-as-tags (an “ordinary” group can be used as tags) then those are not “real” tags and are not retained when a document is copied or moved to another database.