Powermark,
I don't disagree with what jcmkkk said, except on the hard drive issue. IBM HD's have had problems (75 series in particular), but if it's another model, it could be very reliable. I have a 4 year old IBM Deskstar in a secondary computer that has never given me a problem over the time I've had it.
On the other hand, I put together a new system, and used Maxtor HD's, just because of the IBM problem, and haven't had any problems.
As far as the usage question: If you partition your HD, it doesn't matter because a single HD is going to "work" anytime a partition is called on. It's one component being used no matter how many partitions are created.
As far as usage, it really depends on the HD. Not all HD's are created equal. Some hold up to continuous usage fine, while others, mainly drives designed for server app's, are built to withstand 24/7 usage. It really all depends on the HD and the recommended usage.