I always wanted to mix my Time Machine backup with my usual data. The problem is the Time Machine backup will eventually fill up unused space. The usual solution is to create a partition for Time Machine. I came across this how-to article and made my own Time Machine sparsebundle. I set the max size I want. Now I can safely use the rest of the space for other data. The article : http://www.macosxhints.com/article.php?story=20080519051720677