Three drives minimum, no partitions on any.
C: OS, Sonar, all synths (no content, just the dlls any "program" files) and VST.
D: All the project files (which of course contain the audio files)
E: All sample files for the synths on the "C" drive.
Fourth drive (optional);
F: I call this "Studio"; data, finished mixes, Sonar's waveform picture files.
I like to keep the pic files on a separate drive as these are being "drawn" while
Sonar is "working".
If you have "streaming" synths, I would keep them on a separate drive from the "static" libraries.
Tom