Usually the external hard drives have good instructions with them and when you plug them into the usb drive, you computer will recognize it and assign a drive number to it.
Your hard drive is usually "C" drive, you can see when you go to my computer what drives are there after you connect it and do any installing.
You just take the pattern on the screen, go to file in the upper left corner and say save as and choose where you want to save it.
Just remember that all things mechanical fail at some time or another and several back ups are better than one.