DEVONthink stores all data in a dtBase2 file. It's a container which contains the file and other settings. I have a DB which I want to use it from two different accounts on the mac. I created the DB under the first account. So it belongs to that user. It is advised to close the DB before opening by another user. Since the second user does not have permissions, it opens in read-only mode. To fix this we can add read-write permission to the DB file. Now if we try to open it will show Database seems to be already in use warning and when clicked on continue anyway it won't open. The solution as suggested by cgrunenberg is to quit DEVONthink, remove the .dtBase2 extension, which will now appear as a folder. Now add the second user with read-write permission for the folder and apply changes for enclosing files and folders. Add back the .dtBase2 extension. Open the DB and it will work. It's advised to not use the same database by multiple users at the same time.