[dnanian]

Here's what to do:

* Start Disk Utility
* Select the external drive hardware in the sidebar
* Click the Partition tab
* Use Disk Utility's controls to divide the drive as needed, even as a single large partition. Use "Mac OS Extended (Journaled)" as the format type and name appropriately
* Click Options
* Choose the proper partition scheme (GUID for Intel macs) and accept the page
* Click Partition.

You'll want to partition the drive into two (minimum) partitions, one that's 80GB for your Internal, the other whatever size you want for your 'archived' files. You can then use Finder to move them across.