Are you planning on getting rid of your current computer? Are you buying a new computer and planning on trading in the one that you have? What do you plan to do with your personal data stored on your hard drive? If you do nothing with it before you turn your system over to someone else, you may as well open your wallet and hand them your personal papers and ID. Certain precautions should be taken prior to giving up your system to someone, rather it's a friend, a family member or a tech service.
There are utilities that will take care of that problem for you. At D.M.E.I. we make sure that your hard drive gets scrubbed clean as a fresh out of the factory hard drive.
Now that you've prepared the drive for disposal, what is replacing it? Will it be a new system or just a new hard drive. If it is a new hard drive it will probably be best to get the biggest drive that you can. It's a fact that no matter how much hard drive space you have you don't have enough. Some drives may be larger than your system bios can handle. This is especially true if the system you are using is an older system using what we in the tech field call legacy devices. Hard drives today come with software to fool the bios into thinking the drive is smaller than it really is. This software allows you to format the drive outside of the dos footprint but it will still be dos compatible. What will happen is that when you boot up, you will be loading an interface that interprets the drives parameters for the system allowing you to maximize your use of the drive space. The difference is noticed if you try to use the dos formatting utility. You will not be able to format it to it's true capacity. Usually the utility supplied by the drive manufacturer with the drives only work on that manufacturers devices and many times even only on certain drives in the family.