In the past, changing the size of a Macintosh volume has been a significant undertaking, requiring third-party tools, offline reformatting, or both. This tended to complicate storage management, and made it difficult to handle unexpected demand.

Fortunately, all that changes in Leopard. In the newest version of OS X, you can grow or shrink any HFS+ partition, whether it’s a local hard drive or a mounted network array. The resizing is done live, while the disk is online, and can even be performed on a mounted boot volume (though you’d want a good backup before trying it).

Open Disk Utility and select the storage device you’re resizing in the left column, clicking the “Partition” button on the pane to the right. What you’ll get is a graphical representation of the entire storage space, with the portion currently being used in purple and the available space left on that partition in white.

To grow a volume to fill the whole disk or array, grab the diagonal markings on the active partition and pull down until it reaches the size you need. To shrink a volume to repurpose unused space, simply drag upward on the same boundary. If you need to add or subtract empty partitions, you can do so with the “plus” and “minus” buttons. Once you’ve made your adjustments, just click “Apply” and your Macintosh partitions can finally be resized instantly.

Recommended Reading: Back when it was an undocumented feature available only on the command line, Kirk McElhearn wrote about how to resize partitions on the fly with OS X 10.4.6 (and a whole lot of luck) for MacWorld magazine.