Yep, I agree with pixellany. You're probably describing the need for a RAID 1 array. See Wikipedia for a general description of RAID
In this case your friends are probably software RAID
& eventually LVM
. It might become a little bit complicated if the partition you want to "RAIDiate" is as well the root filesystem.
The links mentioned by ehawk concern as far as I understood methods of taking backups/images of an existing filesystem but they won't provide a "live" sync between HDs meaning that once that the active one fails, you'll fall back to a snapshot of the data you took in the past, losing the changes that happened between the failure and the last backup/image creation.