Grant Full Control permissions to the destination folder where the backups are created for the accounts that are running the SQL Server and SQL Server Agent services.
  1. Click Start > Run and type services.msc
  2. Locate the SQL Server service, right click and choose Properties
  3. Choose the "Log On" tab and note what account is being used to run the service
  4. Repeat steps 2 and 3 for the SQL Server Agent service
  5. Browse to the folder where the backups are being created, right click and choose Properties
  6. Choose the Security tab and verify the accounts used for each service has Full Control permissions to this folder
  7. If not, grant Full Control NTFS permissions for these accounts
  8. Perform backup again