well, under linux terms, local should be any harddrive connected to the computer. internal or external.
depending on the kernel your linux os uses, hdds are listed as either sda or hda in /dev/. what you could do, and remember what i said earlier, edit /etc/fstab and add the location to the external drive to the list with the proper settings (format type, location, where it'll be mounted (/mnt/backup/) and other settings. setting it to either automount (if it's always connected, or so as a regular user you could type mount /mnt/backup/, or have it do that as part of the sh script ;)).
here's what I would do under archlinux:
Code:
#!/bin/bash
mount /dev/hdb1 /mnt/backup
tar czf /mnt/backup/backup.tar.gz
now, obviously the code will have issues, because it doesn't have a timestamp at the end of it (which i'm not good at sh programming, but that's a basic almost working example of how it would go ;)
good luck