It has been a year that when I started this thread. I want to update. The cause of the problem was because of a eSata adapter. As soon as I removed it last year and replaced it with a long SATA extension cable, the HDDs never disconnected again when copying large files.
Therefore, it was not a Power Supply Unit or any other issue. Don't use the eSata adapter that comes with the Gigabyte motherboard.
Originally wanted Millenium Falcon as nick but there is character limitation.
Falcon Speed = Millenium Falcon = Light Speed