If I drag a document into DT so that DT is the only place it lives, will making sure the .dtbase2 file is backed up (I use backblaze and time machine) ensure that I can restore that file in the event of a failure. I understand that the DT backup is only metadata and not the actual file.
In the event of a hard drive failure how do the documents get married back together?
If the file is imported (not indexed via File > Index… or via Cmd-Alt drag & drop), then it’s actually inside the .dtBase2 file and a backup of the database contains everything that’s necessary to restore it. However, indexed items require that you back up the files/folders in the filesystem too, e.g. via Time Machine (but not only).