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How to Erase Your Macs Hard Drive Before Selling It (OS 10.4)

Posted 2007-06-29 14:06 in How-to by Ed Shepard

Hannah, the manager of our Burlington retail store recently decided to upgrade her Core Duo black MacBook to a Core 2 Duo black MacBook. First, she used Migration Assistant and a firewire cable to move everything from the old Mac to the new Mac. Migration Assistant is included with OS 10.4 – it can be found in the Utilities folder which is nested in the Applications folder.

Next, Hannah needed to erase the old MacBook’s hard drive, and reinstall a fresh copy of the operating system (OS). The OS installation disks that came with your Mac (sometimes called the Software Restore disks) or a OS 10.4 Install disk is required to do this.

Most people simply install a fresh copy of the OS over the old copy of the OS. This has the effect of overwriting and obscuring / erasing their old data. To do this, pop in the OS install DVD that came with the computer (or a purchased copy of the OS), restart the Mac holding down the “C” key, and follow the onscreen installation instructions – making sure to choose “Erase and Install” when it comes time to actually install the operating system.

However, Hannah wanted to be sure that her old data was really erased. Thus, she decided to first erase the MacBook’s hard drive before installing the new OS. Again, pop the OS installation DVD into your Mac’s optical drive. Restart the Mac while holding down the “C” key. This time choose “Open Disk Utility” on the menu bar from the Installer menu. In Disk Utility, select your Mac’s hard-drive icon in the left column. Click the Erase tab, then click the “Security Options” button.

Now you can choose to Zero Out Data, which provides good security in minimal time, 7-Pass Erase, which writes data over the entire disk seven times (and takes seven times longer than the Zero Out Option), and even a 35-Pass Erase option – which would take a very long time, would be extremely secure, and is great for very paranoid people.

Why the different security options? When a hard drive is “erased” most of the time the old data is not really erased, but set aside to be written over on an as-needed basis. If you never actually write over the old data, it is still there – ready to be recovered by scheming people.

To truly erase a drive so that no one else can recover old data, new data (random data, a meaningless stream of 1’s and 0’s) needs to be written over it multiple times. When erasing a Mac’s hard dive prior to a new OS installation, most people will be fine choosing “Zero Out Data,” and then re-installing the OS. However, doctors, lawyers, accountants, and spies should probably choose 7-Pass Erase before reinstalling the OS.

It does take a couple hours to securely erase your hard drive and install a fresh copy of the OS – but if you have sensitive data, it’s worth the trouble.

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  1. Saw this on TUAW, Your famous now, nice post Ed!


    — Dave Jaques    2007-06-29 16:56    #

  2. Actually, to truly erase a drive, you need to take the approach Wikipedia suggests:

    “If one wants to be absolutely sure that the file is not recoverable by any means, a suggested approach is to incinerate the media (e.g. burning the hard drive to raise the platters above their Curie point). Another approach is to destroy the media with acid.”

    It’s what the NSA does. But I guess that would mean you couldn’t sell your hard drive to Smalldog, eh?


    — mac en tot    2007-06-29 17:07    #

  3. “Zero Out Data” will prevent any software-based file recovery from restoring previous data. There is no way that software can interrogate the hard drive and reveal previous 1’s and 0’s beneath the current 0’s from a “Zero Out Data”.

    The more secure 7- and 35-pass methods can prevent expensive hardware-based recoveries, in which the magnetic moment of each magnetic domain can be measured to reveal information about its history (a 1 bit might be a domain with spin up +1, and writing a 0 bit should flip the spin down to -1, but its actually more like -0.9 indicating that its previous state was +1).

    Users can choose their level of paranoia by asking to what extend the new owner would go to recover the data: inexpensive software or expensive hardware methods


    — Rafe H.    2007-06-29 17:44    #

  4. Incinerating the drive will work as will shredding it. However rather than destroying the disk why not remove it from the system, replace it with a new drive, then take the old drive, and buy an external case for it.

    Delete the operating system and applications from the old drive and use it as your backup media.

    OR

    Format it and use it as a fresh backup disk.

    In either case you should protect it from prying eyes just as you should any backup media.

    You are as careful with the backup media as you are with your system. Aren’t you?


    — DWS    2007-06-30 01:59    #

  5. Great tip! I just tried this—it took me a while to realize that to get the “Options” button to be enabled, I had to choose the icon for the hard disk (e.g., “38.3 GB Maxtor”) and not the icon under it for the logical disk (e.g., “Macintosh HD”).

    (This was on a Mac OS X 10.3 install CD—it might be different in a newer version.)


    — Ned    2007-07-04 17:10    #

  6. Burn iT!


    — beaner    2007-09-08 03:58    #

  7. Verizon is about the only place you can get the authentic RIM product and matching door. Most other sites are out of stock and even when they are in stock they have the black battery door which looks like crap. http://www.batteryfast.co.uk
    I tested this camera for a client. I didn’t have the light running for more than 15 minutes. The battery lasted approximately 6 hours before recharging. The LCD, however, had a few dead pixels – never saw this before. Tried returning for exchange and had to put up quite a fight. Anyone else seen this? http://www.batteryfast.com
    :)


    — lory    2008-02-24 20:44    #

  8. yeahhhh Burn IT!!! Congrats excellent Note & Comments


    — Luca Zaggatii    2008-04-19 18:32    #

  9. Spies would not burn the media, they’d take care of it.


    — machinehuman    2008-04-19 20:41    #

  10. I’m not sure I would want a drive previously owned by the CIA. It’s a nasty business they are in.


    — Troy Banther    2008-04-23 09:06    #

  11. Nice post. You can erase your data by zeroing your hard drive. formatting doesn’t erase data permanently. You can use drive wipe software that overwrite hard drive so it becomes unrecoverable.


    — shrek    2008-06-27 06:59    #

  12. I have a powerbook g4 that I want to get off my hands. One problem, the CD drive doesn’t work anymore. Is there any way to erase everything without having to reinstall OSX from the disc ??????? HELP !!


    — e.    2008-08-06 21:08    #

  13. Agreed, I don’t have the disk any longer, any ideas?


    — chris    2008-08-24 23:09    #

  14. try running the following commands from the terminal. Boot into Single User Mode by pressing cmd -s the geer is spinning. Type the following commands:

    mount -uw /
    find / -user yourshortusername -exec rm -R {} \;
    cd /var/db/
    rm -R netinfo/
    rm .AppleSetupDone
    reboot

    This should take a little while if you have a lot of files. It will delete ALL of the files belonging to the user name that you specify. So you may need to run the find command more than once to get all the users. Next it will delete the netinfo folder, which stores information about the accounts on the computer, and the file that tells OS X that setup of the machine has completed. When you reboot, you should see the Welcome to Mac movie followed by the setup assistant.


    — Devon Govett    2008-09-25 07:07    #

  15. I inserted the install disk, helc the “c” button then went through all of the other steps. When I pushed the erase button to execute the erase I got an error message saying that the disks could not be mounted. can anyone help?


    — Andrew Hagerman    2008-10-28 17:42    #

  16. i am considering using the 7 pass erase on my macs 320 gb hd, i know this isn’t necessary but yes, i am one of those paranoid people. Can someone give me a realistic idea of how long this will take me


    — samuel    2008-12-10 14:25    #

  17. I am currently doing the 35-pass erase and it’s going to take me 1 day and 15 hours :D, I dont want all my banking details going to the person I sell my mac to.


    — ue    2009-01-26 18:35    #

  18. Can one use the OS delivered with a new Macbook (instead of the OS that came with the Macbook one is trying to erase) to install after the erasing?
    Thanks.


    — Sharon    2009-02-07 08:08    #

  19. What if you want to erase an OLD mac, running 8.5 (just held on to it for toooooo long) no longer have the disks for 8.5 and don’t expect anyone will want to use it. Just want to erase the information.


    — Jen    2009-05-03 21:02    #

  20. Hello this is a open Q to any that knows. i have a a macbook and im trying to change somethings around on it when i got it i only put one passy in that i use on it . ok the prob when i got to sys pref. and acc the lock at the bottom left coner i click box opens ask me to enter my admin name and passy i enter it 3 time it give me security question try it again nothing happen ive tryed every passy i ever use and nutin but i can sign on what same passy and it wrks when i try to chang my admin name i can it ask me for pass thats it and i use the pass what i use to sign on to my computer


    — deejay    2009-07-09 22:22    #

  21. What if it stalls during this process? I was doing the 7-pass (I didn’t know if 1 was enough). It ran fine for 3 days, and now it’s been stalled for 3 days. The blue bar is still flickering, but not advancing. Thanks.


    — liz    2009-08-15 11:17    #

  22. A 7 pass erase is going to take 12-20 hours depending on the size of your disk, a 35 pass erase could take days. 7 passes is the MAXIMUM almost anyone would need to do, realistically a zero-pass erase should be good enough for 99.9% of the population. Only sophisticated hardware restore methods would be able to extract data from a drive that has been erased using ANY of these methods, so unless some government agency is after your military secrets and are willing to spend big money to extract information from your drive, don’t waste your time with the 35 pass method.

    Also, if you really really want to be secure without waiting for a 7 or 35 pass, simply zero out the data on your drive (could take an hour), then take your drive out, drill a few holes through it or smash it to hell with a hammer. It will be unrecoverable.


    — Alyn    2009-11-21 22:40    #

  23. Thanks! This was much more understandable than the apple site! I was finally able to erase my HD!


    — Sara    2009-12-30 14:16    #

  24. Thanks! I was able to erase the hard drive on my old MacBook and will finally feel comfortable selling it.


    — Julie    2010-01-29 19:20    #

  25. Thanks dear……..nice explanation…….keep it up…………


    — Ricky    2010-03-02 06:32    #

  26. i am in process of erasing and preparing one of my macs for a clean install of snow leapord. and what i am looking for is to truly free up memory. so if all this data really can’t be erased … there is no true way to free up precious memory.


    — kristin    2010-03-02 12:41    #
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