You do not have access to the folder with your current user - you either need to impersonate a user who does have access permissions, or change the permissions on the folder. The other thing to consider is that since this is in an SQL DATA folder, there is a good chance that the file is going to be in use by SQL when you try to copy it, and the copy will fail because the file is in use.
If this is an SQL database, why not back it up using SQL itself instead? This might help: Backing up an SQL Database in C#
] (it's in C#, but teh code shoudl translate very easily.