Smaller 2.5-inch external drives are typically "bus-powered," meaning they get electricity through the USB cable. Larger 3.5-inch external drives often require an additional AC power adapter plugged into a wall outlet to spin the internal motor.
With the correct hardware in hand, the physical connection process is simple. The user should first ensure the laptop is powered on and booted up. If using a 3.5-inch drive in a dock, the dock must be plugged into a wall outlet and powered on before the data cable is connected. For 2.5-inch drives, the user simply connects the SATA end of the cable to the drive and the USB end to the laptop. In some cases, particularly with older laptops or drives that consume high power, the adapter may have a double-headed USB cable; both connectors must be plugged into the laptop to ensure adequate power delivery. how to connect hard disk to laptop