Seriously.
Very few people have more than 1 hard drive, and viruses typically attack your opperating system, or email, or whatever.
What I'm saying here is, if a virus locks up your computer, or you can't start it, or if you get that crazy virus where the hackers hold your computer hostage until you get a prepaid credit card and send them money... you can simply reinstall your windows backup (if you backup your files), and your non-C drive will not have noticed.
Now if you get one of those viruses where the hacker can get control of your system, obviously what I said earlier is out of the question.
I've managed to get a couple viruses over the years where I had to boot my computer from the CD to an earlier restore point - before i got the virus - just to get around it. If it locks up your OS, and your OS is on your C drive, your E drive won't have noticed.