if the C: drive breaks then so does the D: (and vice versa) so one should not be seen as a backup of the other.
Ah, backups...
I had a hard drive failure a few years back in which I lost a lot of stuff that I wanted and hadn't backed up.
Over a 10 year period, at least 5 colleagues had hard drive failures, including one who lost a year's worth of work. The company had to spend thousands getting the drive rebuilt by data recovery specialists to get some of the data back.
I told my bro-in-law about the importance of backing up his PC and fitted an external drive for him. I've had to restore his system a couple of times in 3 years after PC problems.
I finally decided that I should take my own advice and fitted a backup drive to my PC after Christmas. Since then, my PC power failed at a critical time and my hard drive got corrupted so Windows wouldn't start. I was able to use my backup to get my system running again in a couple of hours with the only data loss being what I'd saved since the backup.
If you only use your computer for fun, don't have any files on it that you'd mind losing, and wouldn't mind having to reinstall the operating system and your applications, then reregistering them, and going through setting up all of your personal preferences again - then, and only then - don't bother doing backups.
If you use your computer for anything serious, keep important files on it, wouldn't like to spend a week getting it back to the operational state it was in before your hard drive failure (that's how long it took me the first time without a backup) -
then buy yourself a backup drive and do regular backups of your complete system! 
