Backup Cant Find Database Name

I was trying to backup my database file using My.Computer.FileSystem.CopyFile but because SQL keeps the file locked this doesnt work (I am using VB.NET 2005 Express and SQL 2005 Express). It seems that the Backup command is what I want to do but it doesnt recognise my database name, which otherwise appears in the Database Explorer tab of the IDE

I wrote a function to use the Backup command as follows
Public Sub MyDbBackup()
Dim sqlconn As New SqlClient.SqlConnection(MyConnectionStringdb)
Debug.Print(sqlconn.Database)
sqlconn.Open()
Dim Backupcommand As SqlClient.SqlCommand = New SqlClient.SqlCommand("BACKUP DATABASE MyDatabase TO DISK = 'c:Mydatabase.bak'")

Backupcommand.CommandType = CommandType.Text
Backupcommand.Connection = sqlconn
Backupcommand.ExecuteNonQuery()
sqlconn.Close()

End Sub

But this gives the error
Could not locate entry in sysdatabases for database 'Mydatabase'. No entry found with that name. Make sure that the name is entered correctly. BACKUP DATABASE is terminating abnormally.

I have also tried with the .mdf extension, with the full path name, and with the name as given by sqlconn.database which is supposed to be the full database name, with and without single quotes surrounding it.

How can I find out which database name this function requires and why it is not finding the database? Other queries on it are working fine.

Thanks

Martin