There are more than one reasons why you may want to go for reformatting a hard drive on a Mac. One of the most common reasons is that you need to sell it. You don't want to hand over a PC, with personal information to a stranger. Another less-likely reason might be an irreparable Mac OS X installation or a virus attack. Though Macs are usually immune to virus threats, there is a growing breed of computer viruses which target MacBooks and Macintosh machines.
If your installed programs are all intact and working well, there is no need to reformat your system, to protect personal information or data. Just create a new 'Administrator' account and log into the system. Then delete all the other user accounts that might have been created on the Mac, after backing up personal data. After all the personal accounts have been deleted, along with the data, the Mac is as good as a reformatted computer.
Procedure For Reformatting
You can format or 'erase' any drive, other than the one which holds the installed Mac OS X, by using the 'Disk Utility' program, which is inbuilt in the operating system. However, reformatting the entire hard drive, can't be done that way. For that, you will need the installation disk of Mac OS X.
Step 1: Back up Personal Data
A reformat of the hard drive or the system volume will completely wipe out the contents of the drive. So it's essential that you back up your data, before you go ahead. An external hard drive is the best storage device, for backup purposes. You could also use a DVD or other optical storage media for the purpose.
Step 2: Format System Drive and Reinstall Mac OS X
To begin reformatting after backup, insert the installation disc of Mac OS X and restart the computer. If the right boot order is set, you should see the set up screen of Mac OS X. Open the utilities provided on the disk and open the 'Disk Utility' in particular.
Then choose the system drive and click on the 'erase' button, after selecting a file system for formatting. Depending on the size of stored data on the disk and its overall size, this will take a few minutes. Once the formatting is done, you can go ahead with reinstallation of Mac OS X, by following on-screen instructions. All you need is the installation disc which has the necessary tools for the job. It is essential that you back up your data, as otherwise, it will be totally wiped out after reformatting.