If you have a spare hard drive from your laptop, which you are now willing to use as an external drive, you will need to get some equipment. Let us talk about the process first. The reason you are going to have the spare drive is because you upgraded the one on your laptop. Now you have two hard drives, but only one can go in your laptop on a permanent basis. The spare one, your older drive, is going to get converted into an external drive thanks to the laptop hard drive enclosure that you are going to buy.
When you get your laptop hard drive enclosure, you can use the enclosure to keep the hard drive safe and to connect it with your computer. For instance, you will take the enclosure, open it up, place the hard drive in its spot, and close it up. Now you will use the USB cable that comes with your enclosure and you will connect it to the computer. These hard drives are generally 2.5 inches, and they do not require any additional power to turn on when you connect them with your computer. All you need is the USB connected to your desktop or laptop.
When you have the connection, you are ready to reformat the hard drive so you can use it as an internal. The reason for the reformat is because you do not need all of those OS files that you have on the hard drive right now. Those are only needed if it is being used as your main drive, but that role is being taken on by another hard drive. So, complete the reformat of the hard drive and it will be completely blank. You will have all of its space, and you can use it as an external with no problems at all.