System.Data.sqlClient.sqlError: cannt open backup device ....operating system error2( error not found )

I am working with SQl server 2005 and I am trying to back up the database using SQL server management studio Express but I encountered this error

System.Data.sqlClient.sqlError: cannt open backup device 'E:........' operating system error2( error not found ).

Even if i try to back up the device on another directory like c fore example I still having the same error .

Please how can I solve this Error ?

Please Help me Since I am new to Sql server ......