When recording audio for a session, we never record to the C drive. Why? In short, because that's where the OS is. It's a resource thing.
So yeah, what Irock said.
That being said, you don't really need a second hard drive unless you're doing intensive stuff like recording raw audio and video, because those things take up a lot of space and resources.
Another example: our school has some gigantic VST sound libraries (if you don't know what those are, they're essentially soundfonts, just bigger, better, and ... BIGGER). These things can be over 100 Gigs total, 100 Gigs of samples of violins playing each individual 7 different ways. Anyway, we keep these either on a second hard drive, or an external hard drive. The reason is very similar to the situation above; loading and playing these sound libraries takes a lot of resources, and if you're loading them from the same place your OS and all your audio programs are, that means there's less "computing power" (so to speak) to devote to playing those gigantic samples. So in this case, using a second drive is highly recommended.
I'm not 100% sure, any maybe Roph knows exactly why, but I assume it's because the disc's laser is going back and forth reading information specific to the OS, the audio program, AND the samples. If it's on two separate discs, the workload is lessened, and information is read faster.