because I can't register to the old forum, I'll post my comment here. Hope that someone can link this topic on the old one :
As the original author, I'm using 2 computers depending on my location. I'm the only developer. Turning one a machine 24/24 wasn't a solution too.
Making a backup/restore when switching from 1 computer to the other one is just the best way to overwrite the wrong database (like anybody has already done in the past with floppy or USB drive ^^).
I can succeed the sync using DropBox 12.4.22.
First computer : Win 7 x64 Pro Fr - MS SQL 2014 Express SP2
Second computer : Win 10 x64 Pro Fr - MS SQL 2016 Express
On the first computer (which has generated the MDF), I've changed the Security Attributes in Windows Explorer of both files Database1.MDF and Database1.LDF to allow my current user the full control. So doing this, let Dropbox sync these files without the error 'Access Denied'.
On the second computer, no need to do this, files will inherit of current folder.
Note : When using VisualStudio, some files are locked with the database and DropBox cannot sync them. Nevermind, these files will be detroyed when VisualStudio will exit.