As I do not know how you have set your bios settings up, I am presumming that you have it set to boot to the main hard drive first. If you install the second hard drive it should just show up in my computer as another hard drive. If it does not then you may have to adjust your settings in your bios. On the other hand it may take you to a boot menu to let you choose which hard drive to boot to if they both have operating systems on them. Either way it will not harm your computer. If you have any problems just remove the extra hard drive and start again.