More hard drive addition questions...

One thing to bear in mind is that read speeds on the outer edge of the disk are considerably faster than the inside edge, so it makes sense when you're partitioning to have your sample libraries on the outside. When you're using partitioning software that displays the partitions as a block, the leftmost side of the block is the outer edge (and I believe Windows gives partitions their letters starting from the outer edge). I have a 2TB WD Black that I use for both sample libraries and backups, and I have the samples on the outer edge and the backups on the inner edge. 

I don't think you'll see any performance gains or other advantages by partitioning, it's really just for organization and convenience. To be honest I think I'd be completely lost if my drives weren't partitioned. 
 
Actually I have a lot more than samples in my sample partition, which I named "libraries." I also have all of my presets in there, as well as software documentation, various installers, Sonar plugin layout files etc etc. I don't have the ideal setup because I only have two drives: a system drive and an audio drive. I also have a partition for audio and projects on the same drive. I don't record audio much (mainly MIDI) and so I have no pressing need to separate my audio and samples onto two different drives. 
 
The backup partition contains all of my backups, including complete system images. They don't really take up much space at all. 

I should really make a separate partition for presets and the like, yes. I didn't divide the partitions equally, I gave the bulk to the sample partition and the audio partition. The backup partition didn't need to be very big. However, it's a very easy process to change the size of partitions if you change your mind later.