For reformatting a hard disk drive for a Mac OS X machine, you have to follow only a few steps. Let us see how it can be done.
First of all, you open the system drive, which will be usually titled “Macintosh HD.” Go to Open Applications – Utilities and then double click at the “Disk Utility” program for opening it. A window will be opened which will be giving a list of all the drives which are currently mounted on the computer. 
Then select the disk which you need to format. It is impossible for formatting the drive with OS installed without the Installer Disk. If you are planning to erase that drive having OS, then you should not forget to back up all the important files from it.
For erasing a particular drive, just click on the “Erase” tab which is present on the right of the list of drives present. Now the window will be display the options for erasing the hard drive and formatting it to work on the Macintosh computer.
From the pop up menu appearing, you can select the Volume Format. Usually it is best to leave that option as the default volume which is “Mac OS Extended (Journaled)”.
If you got other requirements for the disk apart from this, you can select the format which is best suited for your requirements. For any doubt regarding the volumes, you can look at the Apple’s users manual for clarification.
After selecting the volume, you can give a name to the hard drive. Enter a name for the disk in the corresponding field
This name will appear when the disk is mounted on a Macintosh computer.
After naming, just click the erase button. You will see a window asking you to confirm that you need to erase the drive. Click erase button for starting the process of formatting,
Once format is complete, a fresh drive with the name which you entered will be appearing on the desktop.