Erasing the Primary Drive
The best way to reformat a Mac hard drive is from the operating system disk you currently have installed. Using a previous version system installer disk doesn’t usually work well. You have to complete an extra step that does a high-level erase before doing a more comprehensive low-level one. It will also cause some hiccups when trying to install the new operating system. Essentially, to get off to a good start use the operating system that you have all the disks for. On the other hand, if you are upgrading simply use your new disks.
Why Reformat you Macintosh HD (hard drive)
We tend to reformat a hard drive to prepare the disk for use, either with an operating system or as a storage drive. A corrupt operating system (OS) might also require you to reformat the drive before reinstalling OS X 10.x. This is a drastic last step that you should only use once all other options have been exhausted. Troubleshooting this problem of formatting disk structure damage involves two types of repair paths that use the Disk Utility to address the potential damage to the logical or physical formatting structures. Both eventually lead to the reformatting of the hard drive. For the sake of keeping this to the point I am describing the final step in both of these repair paths.
There are two types of formatting that can be performed on your hard drive: low-level and high-level formatting. Both will erase your Mac hard drive, but there are differences which will determine which type you want to use. The Low-level format is used when you are deleting personal information from your hard drive and preparing it for re-installation of the OS; whenever a high-level format path my have failed to resolve problems that you might have encountered; and when disk testing applications within Apple’s utilities have failed, such as Apple HD SC Setup, Internal HD Format, Drive Setup, Disk Utility, etc. The High-level format is used anytime you are troubleshooting for systems that deal with formatting structure damage. For more information regarding the two complete repair paths please visit Apple’s Support page Troubleshooting Hard Drives: Reformatting
Reformat you Macintosh HD
A high-level format can be used to Erase and Install or just simply Erase your hard drive. This does not zero out, or reset the hard drive’s 0 and 1 binary digit data. It simply overwrites it as it needs space.
The low-level format option you have available to you depends on which version of Mac OS X you have the installer Disk for or have installed on you computer. Earlier versions of OS X prior to OS 10.3.x offered a Zero All Data, returning all the binary digits of the hard drive to 0. Zero All Data does pretty good job of completely erasing your hard drive. Mac OS X 10.4 and later include additional secure erase options that include the once, seven times, or 35 times rewrite options. Note that if you have a large disk overwriting the free space several times can take a long time.
1. To erase the primary drive, insert the OS X installer disk.
2. Reboot the computer. It will ask for your administrator user name and password.
3. Select you language.
4. From the menu select Utilities > Disk Utility.
5. Select the disk you want to erase and click Erase.
6. Specify a volume format and name for the disk.
7. Click Security Options and select your formatting option.
8. Click OK.
9. Click Erase.