Thank you Joma90. I guess it is the answer. I had to replace my 1st drive with the new drive and do the setup with one drive from start. Then I put 1st drive back to 1st tray, and new drive to 2nd ray and voila, the NAS recognized the new drive, but it processed to format the new drive, again! It seems stupid, but it works for now.
But sorry for my stupidity, since my HDD is already formatted, how can I turn it back to the un-formatted condition, except for the silly way I have done?