If you only have one hard drive, and it dies, you will lose everything.
If you have two, you can store an image of the operating system on the second drive. You can put the pgefile on the second drive. You can put backups of your data on the second drive.
Then, if one of the two drives dies, you can still salvage a lot of your data.
Statistically speaking, chances of two hard drives in a system both failing at the same time are extremely low. Much less than the chances of one hard drive failing.
If you are into investing, you may have heard advisors saying you have to diversify your investments and not put all your savings into one stock or Mutual fund. This is just like that.
Noise in a computer is usually determined by the noisiest component in the box. So, if your CPU fan is the noisiest thing and you add a hard drive, unless, the hard drive is louder than the CPU fan, the overall noise of the system is not going to change.
Most motherboards allow you to connect each HD to a dedicated channel. Then, you have two data channels and two heads (each HD has a head). That is faster when you copy data from one HD to the other.
If you only have one HD and copy data from one folder to another, the read and write functions will have to compete for the head since one drive has only one head.
http://support.microsoft.com/d...x?scid=kb;en-us;314482