I have two 1TB My Book Essential drives. They often disconnect themselves from my laptop, stopping any on-going jobs. Particulars – brand new HP Envy laptop, 8GB ram, 2.6 3rd generation i5 processor, 3 USB 3.0 ports.
Cables are brand new.
More info: Moving the drives to another pc/laptop has no beneficial effect.
WIthout moving or touching anything, the drives can drop off. Sometimes they re-connect. Sometimes I pull the USB cabkle from the laptop, wait a bit and then re-insert.