DEVONthink Pro databases are technically packages, folders that appear like a single file. When you simply copy them to another volume as a backup, the Finder always copies the whole thing. And so even do some backup applications, even though only parts of the database have changed. Try ChronoSync. It has the option to treat packages like files which allows it to peer inside the packages and only update the files that have actually changed. Command line fans will want to have a look at rsync (part of Mac OS X,) which intelligently copies only those parts of one or more folders that need to get updated.