I use a small software called "Almer Backup" for exactly the same.
I configure it to backup all info every 12 hours to the other HDD. It has also a "speed priority" option, that is very nice if you don't want to turn your PC slow for a few minutes, the program will copy a bit more slower but you can use it without problems.
http://www.almersoft.com/products/backup/
Hope to be useful
