I'm not sure if I understand what you are after exactly but if you want to simply create a backup of your hard disk onto another hard disk on the system you have a few options.
You could set up a RAID so that all data is written to both disks all the time leaving you with an exact copy of your primary disk.
If you want to back up certain directories onto another hard disk you can use mirrordir
to simply mirror a dirctory to the other hard disk. This can be setup in a cron job so that you get backups of certain directories at a specified time.