I recently realized that all my contacts in my address book are gone. I was wondering how I should go about getting them back. I figure I need to use the back-up hard drive for this, but don’t know what to do exactly.

Exactly. As long as you haven’t backed up to the external since the contacts went away, you’re good. So:

1. Quit Address Book
2. In your home folder on your laptop, go to ~/Library/Application Support (where “~” is your home folder).
3. Inside there, change the name of the folder AddressBook to “AddressBook old” (no quotes)
4. Now plug in the external drive, and from your backup’s Application Support folder, drag that AddressBook folder into your laptop ~/Library/Application Support.
5. Eject the external (just in case)
6. Open Address Book. Your contacts should be there.

BTW, if that drive is the LaCie I remember it to be, with USB in addition to Firewire, then with the new Airport Base Station, we can plug the drive into the Base Station and share it to your network. Instant Network Attached Storage. Sweet!