First it somewhat depends on how big your SSD drivers are. And the speed of your regular HD.
If your not going to use the computer for much else you can install Sonar to the C drive and even the content will fit. I only have a 128 GB C drive on my DAW and its still is at about 50% I have 3 versions of Sonar and a bunch of other software on it as well, But I do not store any Data on the C drive. As Bitflipper said I leave Sonar to store everything there, I only park 3rd party Libraries elsewhere.
I use a second 128 GB SSD drive for Data and my working files in Sonar. Projects load nice and fast with the SSD drives.
Then all my back ups and some VST libraries are on a 7200 1TB 3rd drive .
Most VST installs including Sonars will allow you to select a pathway for the content during install so it's up to you.
If your regular HD is 7200 RPM then your good to go with sample libraries but I won't use it if it's 5200.