I have an Umbraco application running on a webserver and I need to move the application to another webserver. The Umbraco application uses an embedded database (SQL CE 4). I can copy all the files in the application except the "Umbraco.sdf" file.
Add file named (if you like you can insert HTML that will be your maintenance page)
app_offline.html
After that IIS will release Umbraco.sdf and web.config copy and delete/rename mentioned file.
As the Umbraco installation will be using the Umbraco.sdf database, you need to turn the application into maintenance mode, by adding a app_offline.htm to the root of the webserver.
You need to make sure it is a .htm-file, i.e. not .html, cause otherwise it won't work.
Also - make a change in your web.config file, to restart the application pool, so that it may notice the change in your site.
Doing so, will release the Umbraco.sdf file, and make it possible for you to download it via simple FTP.
I just had the same issue and didn't want to stop the site to release the lock on the file, so I did the following:
RDP onto the server, make a copy of the file in the /app_data directory (call it something like "Umbraco-copy.sdf")
Download "Umbraco-copy.sdf"
Rename "Umbraco-copy.sdf" back to "Umbraco.sdf"
Perhaps, this error related to permission. You must login as a root (administrator)!
来源：https://stackoverflow.com/questions/13121280/cant-copy-umbrao-sdf-from-webserver