I'd actually say more!... like three
Hard drives are relatively cheap nowadays, and it's good practise to have more than one, as a priority, two would be nice, but three(or maybe more?) would be ideal.
Two HDD's give you the flexibility to use one as a dedicated cache disk, which should help for running your editing program faster. If it's large enough you could always use it for storing gigabytes of data/image/etc...
I have three hdd's on my current system(actually four, if you count the primary Windows drive as it's in a RAID config), but I use the main drive only for OS and programs, second drive for storing stuff images, videos, documents... etc, and the third drive is generally for dedicated cache file use, but as it's large enough it also has some room for storage too, although I only use that temporarily.
Further to all that, I also recently got an extra 1Tb external hdd for added archiving, and have a regular routine of backing up the storage drive to the external drive at 10PM every night.
As I see it for now, the only disaster I'm not
really taking into account is fire.
hdd's are cheap, and the more the merrier.