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My mac has several problems, running very slowly being the greatest. I have decided for that reason and a few others that it will beneficial to format my Mac.

However, I do not have the installation disk.

So the question question remains as is the title:

How do I format my Mac without the installation disk?

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Without the disk there is no way to reinstall. Maybe apple can get you a replacement or you can take it to the genuis bar. –  Daisetsu Jan 3 '11 at 22:27
    
any suggestions with how to improve performance then? –  RSM Jan 3 '11 at 22:28
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Create a new user account in System Preferences. If that one is also slow, it's a system/hardware issue. But if that account is faster, you have too much crap in your user profile. Uninstall software, remove Login Items (also in System Preferences » Accounts), remove Dashboard widgets, menubar utilities, etc. Safari also doesn't like lots of bookmarks, for example. –  Daniel Beck Jan 3 '11 at 22:33
    
Not that it answers the questions directly, but I had slowdowns with my MacBook Pro, and it turned out the be the hard drive. I downloaded SmartUtility on the recommendation of Spiff's answer in superuser.com/questions/357308/… and I haven't looked back –  Canadian Luke Jan 11 '12 at 22:03
    
How does this question have 150000 views, yet isn't highly upvoted or has an accepted answer. Good job, SU. –  ekaj May 21 '12 at 4:00
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5 Answers

10.7 and later

You can can erase or reinstall OS X from the recovery partition. See http://support.apple.com/kb/ph11273.

  • Hold command-R on startup and open Disk Utility.
  • Select the main volume (like Macintosh HD, shown indented below a drive). Erasing the whole drive would also remove the recovery partition, and the Mac would have to start up in internet recovery mode.
  • (Optional) HDDs can be erased securely by selecting one of the options in Erase > Security Options. Secure erase is disabled for SSDs, but you can encrypt the drive with FileVault before erasing it normally.
  • Open the Erase tab and press the Erase button.
  • (Optional) Choose Reinstall Mac OS X to perform a clean installation.

Choosing Reinstall Mac OS X without erasing the old installation just upgrades the old installation.

10.6 and earlier

If your Mac meets the system requirements for 10.8, you can install 10.8 from App Store. It always creates a recovery partition, so you can follow the instructions above after that. If you don't have the App Store application, upgrade to 10.6.8 first.

Otherwise you can buy a used installation disk or borrow someone else's disk. The disks that came with Macs did not always work with all types of Macs, but the disks that were sold separately did. You could also just download a torrent and burn an installation DVD or CD yourself.

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I have a MacBook Pro that was originally a machine from an old company so I didn't have the original disks either and I too was looking for a way to clean off the machine. I found this blurb in the apple forums that discusses a few options without disks.

3 ways to go...

  1. Insert the Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.
  2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.) Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
  3. Click the Erase tab.
  4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
  5. Select your Mac OS X volume.
  6. Highlight the drive, select Partition Tab, then Format type... MacOS Extended Journalled, select the Security Options button, choose Zero Out Data, Erase... after completion do a new install.

Or...

Open System Preferences>Accounts, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

In the same pref pane highlight your old account, click the little minus icon, then use Disk Utility to Secure Erase Free Space.

Or...

If 10.7.0 or later...

Bootup holding CMD+R, or the Option/Alt key to boot from the Restore partition & use Disk Utility from there to erase & reinstall.

https://discussions.apple.com/thread/4039984?start=0&tstart=0

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Welcome to SuperUser. Please do not just post a link, copy some of the highlights and post them here as well. –  Kruug May 9 '13 at 13:30
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To more properly remova all user data, I would follow the instructions described here:

  1. Boot into single user mode. Hold Command-s at startup.

  2. Check the filesystem:

    $ /sbin/fsck -fy
    
  3. If no remaining errors, mount the filesystem:

    $ /sbin/mount -uw /
    
  4. Start up directory services. For Mac OS X up to 10.6:

    $ launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist
    

    For Mac OS X 10.7:

    $ launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist
    

    Note that single user mode said to use com.apple.DirectoryServicesLocal.plist, but that didn't work —this did.

  5. Find what groups the user belongs to:

    $ dscl . -list /groups GroupMembership | grep username 
    

    And then remove your username from the groupname:

    dscl . delete /groups/groupname GroupMembership yourusername
    

    Repeat for each group except for the user's own group.

  6. Remove the group corresponding to the username:

    $ dscl . delete /groups/username 
    

    (this may not be — you may get an error that the group doesn't exist; you can ignore it and go on).

  7. At this point, you may wish to remove or archive the user folder in /Users. To remove the user account:

    $ dscl . delete /users/username
    
  8. You may wish to remove the .AppleSetupDone file in /var/db to cause the Setup Assistant to run when next booted (aka Out Of Box Experience).

    $ rm /var/db/.AppleSetupDone 
    
  9. All done? Type $ reboot to reboot the system or $ shutdown -h now to shut down the system.
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I have (several times) ordered replacement install disk from Apple. Call them and give them the serial number of the machine (or machines). Last time I ordered (for my wife's used MacBook Pro), it was $25 a disk, and her machine needed two disks.

I have been able to get replacement disks for machines as early as G4 Macs.

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You could create a new administrator account and trigger the OOBE (Out of box experience) setup. I can give instructions if you're interested.

  1. Reboot
  2. Hold apple key + s key down after you hear the chime. (command + s on newer Macs)
  3. When you get text prompt enter in these terminal commands to create a brand new admin account (hitting return after each line):

    $ mount -uw / 
    $ rm /var/db/.AppleSetupDone 
    $ shutdown -h now
    
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Yes please. I would like to know more! –  RSM Jan 3 '11 at 23:23
    
Added to original answer –  hyperperforator Jan 3 '11 at 23:27
    
Do tell -- what other OSX setup is there than an OOBE setup? ;) –  Matthieu Cartier Jan 3 '11 at 23:28
    
You make a fair point sir, I just felt like saying fancy words ;) –  hyperperforator Jan 3 '11 at 23:28
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Fixed the code tag (after an ordered list you need 8 spaces). The OOBE won't change anything on the hard drive except creating a new user account, which you can easily do in System Preferences. It is neat if you're selling your computer or something. –  NReilingh Jan 3 '11 at 23:59
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