Connect the USB cable from the USB external hard drive to the laptop. If there is a power cable for the USB external hard drive, connect the power cable to the USB hard drive and a power outlet.
Wait for the operating system to mount the file system from the USB hard drive. The operating system will let the user know when the newly added storage is available for use.
Select the USB hard drive. On a Windows laptop, click the "Start Menu," then "Computer." The USB hard drive should be one of the choices. On other operating systems, the procedure is similar.
Article Source: ehow.com