[dnanian]

Yes, you're going to want to partition in this case:

* 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. Use "Mac OS Extended (Journaled)" as the format type and name appropriately
* Click Options
* Choose the proper partition scheme (GUID for Intel macs, Apple Partition Map for Power PC) and accept the page
* Click Partition.

As far as "not updating Time Machine" -- you can, of course, turn it off and then turn it on again later. But its whole point is to have it update continuously, so I wouldn't recommend turning it off if you want it at all...