An unheralded new feature Apple quietly added to OS X 10.5 Leopard is the ability to create, expand, or shrink hard drive partitions without erasing the entire drive. Several third-party applications can repartition drives as well, but it is a function that had never previously been built into the Mac OS.
Provided there is open, unwritten space left on your hard drive, using Disk Utility you can add a new partition, remove a partition thereby merging two adjacent partitions, or shrink an existing partition.
You can, in most instances, also achieve more flexibility in resizing or creating new partitions by running a disk defragmentation/optimization using a third-party utility like AlSoft DiskWarrior or ProSoft’s Drive Genius, the latter of which also incorporates a full-featured disk reformatting function and many other drive maintenance, optimization, repair, and troubleshooting features.
I used Drive Genius to repartition the hard drive in my G3 iBook a few years back, needing more breathing space on the primary partition with introduction of OS 10.4 Tiger. The entire operation went smoothly, I encountered no problems, and the computer is still in daily use by my wife.
The latest Drive Genius version 2.1 has, among other enhancements, added DriveSlim, a feature that can free up hard drive space by identifying unnecessary files cluttering your hard drive and allowing you to mark them for deletion.
A Slim Plan Options menu lets you, by adjusting sliders and checking various options, fine-tune your data reduction and filter out specific duplicate or unnecessary files that may be bloating your hard drive, after which you may not need to repartition.
Incidentally, Drive Genius is currently offered till December 18 as part of the MacUpdate Holiday Bundle of 10 applications. The bundle has a retail value of $450, but is available for $49.99, or half the regular price of Drive Genius alone.