How to disable close button?

you can try another option: whenever a user click "close" button, check if you are backuping or doing other jobs using FormClosing event. The following is an example posted on the web for your reference. It is written in C#
----------------------------------------------------
private
void Form1_Closing(object sender, 
System.ComponentModel.CancelEventArgs e)

{
   // check if you are backing up something

   if(MessageBox.Show("Are you sure you
want to exit?", "Confirm exit", MessageBoxButtons.YesNo,
MessageBoxIcon.Question) == DialogResult.No)

   {// ignore close behave

   e.Cancel = true;

   }

}