I currently have a 500gb Seagate for my OS and a WD Black Caviar 1TB for my DAWs/VSTs/Samples/Project folders. I'm  only using a little over a third of the 1tb drive but just bought another program that promises to be somewhere in the 300-350gb range for full install. I have a third slot in my tower and am strongly looking at getting either a 3tb or 4tb WD Black to add to it.
Has anybody set one of these up; and if so, did you do several partitions on it or just one?
I'd like to keep the OS on the first and use the second just for DAW & project files.  Anything I should know about moving the VSTs and Sample libraries and how to best organize them?

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. 

Do you have your 2tb drive partioned as 2 HDs with Samples on one and backup on the other?
I'm thinking about putting samples on one, VSTs patches on another and a smaller partition for my 'Install Folder' that contains all the zip and decompressed folders to install from along with the necessary numbers at hand to complete if I need to re-install. Is putting the samples on a different partition than the VST/patches have any advantages except for organizing or would that make it less effective in the DAW? And I am assuming that backups are for project files or do you backup the entire system?

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. 

Thanks for your replies, they do help.
If I'm following you right, you say the Samples should be to the first block of partition  so that's where I wonder if separating the vst program patches on the next partition might help keep the sample libraries grouped to the first partition without the patches breaking up the files? Just kicking around that idea really.
But more on topic - Did you divide the partions equally or give more room to libraries or more to backup image?
I'm thinking of giving the larger partition to samples for growth, since they seem to be accumulating.
 

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.