How to repartition a Snow Leopard drive

Erik Eckel reminds administrators that it is possible to dynamically repartition a Mac drive (created with OS X 10.5 and higher) without destroying the existing partition's data. Here's how to do it and what to look out for.

Many Mac administrators are surprised to hear that Mac partitions created using Mac OS X version 10.5 and later, including Snow Leopard, can be repartitioned without destroying the existing partition's data. As with most everything, there are a few catches. One, some partitions may not possess sufficient free space to perform the dynamic reallocation of data required to repartition the drive. Second, drives formatted with the master boot record cannot be dynamically reformatted. Such drives have to be repartitioned the old-fashioned way: backing up the data and settings and performing a destructive reformatting from scratch.

Even with those caveats, smart technology professionals will note that dynamically repartitioning a hard drive isn't a perfect process. You should always, without exception, verify a working backup of the data before performing the repartitioning tasks.

The steps to repartition

Here's how to dynamically repartition a Snow Leopard drive:

  1. Begin by closing all open applications.
  2. Connect the drive to be reformatted.
  3. Open the Disk Utility, reached by accessing Applications/Utilities/Disk Utility.
  4. Using the left-pane window, select the drive to be reformatted. Review the drive's size, manufacturer and model number information to confirm you've selected the proper drive.
  5. Using the right-pane window, select the Partition tab. Existing data appears color-coded blue. Free space appears color-coded white.
  6. Resize the volume by clicking and dragging the drive's bottom right corner. Adjust the drive to possess the new size you wish for it to possess. Add a new volume by clicking the + sign icon found in the bottom left corner. When adding new volumes, you'll be prompted to provide a name for the volume and specify its formatting. Alternatively, you can remove a volume by highlighting it and clicking the - icon found in the bottom left corner. When deleting volumes, you will be prompted to confirm the change to prevent accidental deletion of a volume.
  7. Click the Apply button to apply the changes (but note that, when deleting volumes, they are deleted immediately upon confirming the deletion request as described in the previous step).

Alternative method

That's the official method for repartitioning Mac Snow Leopard drives. Other options exist, however. While Disk Utility provides a clean interface that's straightforward and easy to use, what about those occasions when you're trying to repartition drives that run into trouble using Disk Utility or don't meet its requirements? In those cases, you can turn to an unofficial (as in not supported by Apple) disk partition application.

I have successfully used Coriolis Systems' iPartition. The partitioning software enables creating a boot DVD you can use to repartition HFS+ and even FAT and NTFS partitions nondestructively. The tool can also convert disks from Mac to PC and from PC to Mac and works with Boot Camp. For less than fifty dollars, the software provides an excellent method for repartitioning existing volumes.

When using the software, or any third-party disk applications, be sure to review the manufacturer's requirements and recommendations. For example, Coriolis Systems recommends first performing a defragmentation of the volume before reformatting. The company's iDefrag2 makes the task simple.

Just be safe. Always ensure you have a working backup before performing any disk partitioning tasks.