WD my passport 1TB automatically disconnecting

I just bought WD my passport 1 TB.
I am trying to back up my files but when the files are transfering computer makes the sound like when usb device is disconnected and hdd stops workig. like its been unplugged but it was not. what should cause this ? It is pretty annoying when i try to copy my files all over again. 

i am using asus n53sn notebook with windows 7 home premium

Might be a power issue, be sure to connect the USB cable directly to the PC, avoid extension cables or USB hubs.