Beware of virus problems. A 2nd HDD in your machine gives zero
protection to your data if a virus gets it. Many viri can be removed,
When you back up off the machine, again if you get a virus then
backup, and thats the only backup you have, youre stuffed. So always
keep 2 generations of backups, with the older one being say 2 weeks
old, not 2 days. That gives you enough time to spot any heavy handed
viri, and still have good clean data.