You need to check one little box and this error will go away.  Here is how I move my database.

I make a backup of my database and save it as dotnetnuke.bak.

I open up SQL Server Management Studio and created a new database.  

I then right click the database.

Go to Task and then Restore, database.

Browse and select your backup file.

Then click options in the left hand side of the screen.

Check the box that reads, "Overwrite the existing database"

Click OK.

Should work like a charm.