Ideally you should have one hard disk for your OS (Operating System) and one hard disk for your data. But if you only one hard disk it is extremely important that you divide it into two partitions. The Reason for this is that your OS may crash. And the only fix would be to format and reinstall your OS. You can't do this without losing your data if they reside on the same partition. So as a rule, always...always make atleast two partitions if you only have one hard disk. If only I can remember that rule. Ugh.