In this example, we will partition an external hard drive (the 1 TB CalDigit drive in the screenshot) into two partitions. Here is the tutorial on how to partition the startup drive while it's being used to boot your Mac.
Once you’ve copied your data, go to /Applications/Utilities and open Disk Utility. If you are using the Mac OS X Installation disc as the startup disk, click on the Utilities menu and select Disk Utility.
The left column lists the Mac’s storage devices. Listed under each drive are the partitions. For example, in the screenshot here, the first drive listed (320.07 Hitachi) is the internal drive of my iMac. Listed underneath that is the one partition that’s on the internal drive called iMac. The second drive listed (1 TB CalDigit) is an external hard drive with a single partition called 1TB External. The last device listed is the internal optical drive.
There is a set of tabs across the top right part of Disk Utility. To partition a drive, the first step is to select the drive you want to partition in the left column and click on the Partition tab.
The drive is selected in the left column, and I clicked on the Partition tab. Disk Utility shows the current partition of the drive. In the next step, we will define the Partition Layout so that two partitions will be created.