The database 'C:\SOME\FOLDER\MYDATABASE.MDF' cannot be opened because it is version 655. This server supports version 612 and earlier. A downgrade path is not supported.
BackgroundI had created that database in Visual Studio 2008 (File → Add New Item → SQL Server Database) and now it wouldn't let me open it!
What I had done is opened, edited and closed the database file using SQL Server 2008 Management Studio. Shouldn't be a problem, right, since I'm also using Visual Studio 2008?
The ProblemWell, although I currently have SQL Server 2008 installed, I also have SQL Server 2005 (which was installed with Visual Studio 2008 originally). When I created the database originally I just assumed it was a SQL Server 2008 database - but it wasn't (it was 2005)!
Turns out that when you are create or open a file-based (.MDF) database (as opposed to using a database instance or server connection) Visual Studio needs some 'help' knowing what tools to use.
When I edited the *.MDF file in SQL Server 2008, it made some fundamental "changes" to the file (even though I didn't 'upgrade' it) which prevented SQL Server 2005 from opening it. Why was Visual Studio 2008 confused? It's a configuration setting!
To fix the problem, go to Tools → Options... → Database Tools → Data Connections and change the SQL Server Instance Name from whatever it was (probably a SQL Server 2005 instance) to your SQL Server 2008 instance, hit OK and you should be fine!
Thanks to simple-talk; and SQL 2008 Express edition error/Add new item on ASP.NET forms. A bit OT - HTH someone...