|Actually Mirror it a term used for 2 drives in Raid together that always copy each other (pretty rough explanation). You really mean Clone which is to copy one drive to another.|
ALL hard drive mfg's offer software free to do this when you purchase their hard drive. If you purchase a 'boxed' version, it is included in the box, but if you purchase an OEM version because it is cheaper, you need to download it from their website.
If this is for a laptop, this is probably the best option, though you can also purchase a portable hard drive and move many of those important but rarely used files onto it.
If this is for a desktop, you also have the option to purchase a second hard drive and install it AS a second drive (much easier) and moving all of your personal files from the first drive to the second drive. You also can uninstall programs from the computer and reinstalling them as custom and choosing the second drive to install them (you cannot just move programs or their components or they will no longer work or completely uninstall).
You have to be a little bit crazy to keep you from going insane.