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I have some problems that i cannot resolve on my iMac and now I would like to delete everything and start it from scratch like when i first bought it a few years ago. Please show me the steps.

Thanks,

Jon

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What model is your Mac, what OS are you looking to install? –  conorgriffin Jan 30 '11 at 23:10
    
It's an iMac Leopard with Processor 2.4 GHz Intel Core 2 Duo and 300GB drive, 4GB memory. –  Jon Jan 30 '11 at 23:17
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should i do the reinstall with erasing and formatting disk option –  Jon Jan 30 '11 at 23:19
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Yes, that's the right way to do it. –  user479 Jan 30 '11 at 23:39
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@Jon, post that as an answer. –  Nathan Greenstein Jan 31 '11 at 0:42

3 Answers 3

There's a nice write-up on TUAW on how to do this:

Erase and Install

This will erase the contents of your drive, format it into HFS+ and install Leopard clean. EVERYTHING ON YOUR HARD DRIVE WILL BE ERASED. If you choose this option, it is crucial that you back-up all of your data safely before the installation. If you select this option, you can click on an "Options" button that will give you additional choices, for instance, removing language files you may never use -- but if choices make you uncomfortable, just go for the standard installation.

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To reinstall Leopard visit this Apple Support page or follow these steps:

  1. Insert the Mac OS X Install Disc and double-click the Install Mac OS X icon.
  2. Follow the onscreen instructions. In the pane where you select the destination disk, select your current Mac OS X disk (in most cases, it will be the only one available).
  3. Click Options. If you want to save your existing files, users, and network settings, select “Archive and Install,” and then select “Preserve Users and Network Settings.” If you want to erase everything on your computer and reinstall Mac OS X, select “Erase and Install.” You can’t recover erased data.
  4. Click Continue.
  5. Click Customize to select which parts of Mac OS X you’d like to install, or click Install to perform a basic installation (recommended).

To reinstall Snow Leopard visit this Apple Support page or follow these steps:

Important: This procedure will completely erase your hard drive. If you have installed Microsoft Windows using Boot Camp, or have multiple partitions, this procedure will erase the partition you select.

Part One: Erasing the hard drive

  1. Turn on the computer.
  2. Insert the Mac OS X Install DVD into the optical drive, or insert the MacBook Air Software Reinstall Drive into the USB port.
  3. Press and hold the "C" key while the computer starts up. The computer will start up using the Install media. Alternatively, press and hold the Option key at startup to select the volume containing the installer.
  4. If your are using a wireless mouse, a mouse icon should appear asking you to turn the mouse on so that the computer can discover it.
  5. Select a language, then press the right arrow key.
  6. Click the Utilities menu, the choose Disk Utility.
  7. Select a volume to erase (usually called Macintosh HD).
  8. Click the Erase tab.
  9. Click Erase... Important: This step will completely erase your hard drive. If you have installed Microsoft Windows using Boot Camp, or have multiple partitions, this procedure will erase the partition you select.
  10. A dialog box appears asking if you want to erase the partition: click Erase.
  11. After the partition is erased, choose Quit Disk Utility from the Disk Utility menu.

Part Two: Installing the operating system

  1. When the Mac OS X Snow Leopard Installer screen appears, click Continue.
  2. When the license agreement appears, click Agree.
  3. Select the disk you erased. It should now be highlighted with a green arrow.
  4. Click Install. A status bar appears on the bottom of the screen while Mac OS X installs.
  5. An Additional Information screen appears, advising you that you operating system has been installed. Click Continue.
  6. An Install Succeeded screen appears. Click Restart.

The OS is now reinstalled, visit this support page to see how to reinstall your applications and configure your user account etc.

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The easiest way is as follows:

You will need the original (or a newer) Mac OS X install disk for the computer. Do not use and older version than originally came with the Mac, and do not use the install disk from any other model of Mac.

Insert the disk and restart the computer pressing the Option key as it restarts. After about a minute when it loads, click the arrow under your installer DVD to boot from it.

When it starts, click continue on English. In the menu bar, select Utilities and select Disk Utility. Disk Utility may be under a different menu on some install disks.

Select your disk in the left column (select the disk, not the partition (select the one labeled something weird, not "Macintosh HD")) and follow the instructions to erase the disk. For best results choose the option to "securely erase" the disk, or to overwrite the data with 0's rather than just mark as free space. It will take longer but is more secure (if your data was valuable) and cleans the disk better. Then close Disk Utility and install the operating system on the newly erased disk.

Viola! Brand new Mac OS X. Enjoy the startup video ;)

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