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I'm selling my MacBook and would like to wipe the hard drive in such a way that the current data can't be easily recovered after I reinstall OS X.

Can you recommend a free tool to help me do this?

Maybe I should mention it's an Air model, so something that's safe to use on an SSD.

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4 Answers 4

up vote 8 down vote accepted

As current hard drive-oriented techniques for file sanitization are ineffective on SSDs, I recommend to encrypt the whole hard drive using Filevault 2 (best if you did this before you put your data onto it).
(This is only possible in Lion, Filevault 1 in older versions of MacOS will only encrypt your home folder.)

This way you will not erase your data, but make them inaccessible without the password.

Your buyer will then have to reformat and reinstall MacOS for his use.

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Won't repartitioning work the same way as just writing more data to disk? SSD might put data of encrypted partition in different place than original data. –  liori Jan 21 '12 at 18:31
    
@liori This does not matter here because I'm talking about whole disk encryption. –  gentmatt Jan 22 '12 at 8:12
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Encrypting an SSD after data has been written to it is no better than overwriting with random data -- many unencrypted sectors will be left behind (but inaccessible unless you bypass the flash translation layer), exactly like a 1-pass random overwrite. –  Gordon Davisson Jan 22 '12 at 9:26
    
@GordonDavisson This is interesting. Do you have numbers as to how much space the unencrypted sectors make up when applying full disk encryption? –  gentmatt Jan 22 '12 at 9:32
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@gentmatt: up to 28% in consumer disks, see en.wikipedia.org/wiki/Write_amplification#Over-provisioning. Also encryption just overwrites blocks on logical layer (that controls wear-leveling), it doesn't have access to physical blocks on SSD -- so you have no control on which physical blocks are overwritten. –  liori Jan 22 '12 at 12:09
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From the Apple Support pages Securely erasing a disk

To securely erase a disk or partition: In Disk Utility, select the disk or partition to erase, and then click Erase.

  • Specify a format, and enter a name for the disk.

  • Click Security Options and choose to write over the data once, 7 times, or 35 times. Click OK.

  • Click Erase.

Writing data over the data 7 times meets the U.S. Department of Defense (DoD) 5220-22-M standard for securely erasing magnetic media.

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Thanks, that's convenient. I see it says magnetic media. I wonder if I have to do this for an SSD. –  Louis Jan 21 '12 at 15:13
    
@Louis you cannot securely delete an SSD with MacOSX. However, if you have Lion installed you could encrypt the whole hard drive. Then let the buyer format the drive again and install the OS himself, maybe? –  gentmatt Jan 21 '12 at 15:21
    
@Louis Check out this post: apple.stackexchange.com/a/6287/13414 –  gentmatt Jan 21 '12 at 15:23
    
@gentmatt thanks, I think I'll encrypt the drive and then reinstall OS X. Would you mind making an answer? –  Louis Jan 21 '12 at 15:49
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Our Enterprise uses WipeDrive and it allows for DoD wiping. The newest version appears to work well for SSD's, as well. I believe wiping SSD's is still a gray area, though.

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For an SSD, I recommend Apple's Remote Wipe, accessible via Find My Mac. That's Apple's best engineering answer to how to wipe the drive and it will have an awareness of the special challenges regarding wiping an SSD, at least if you're using an Apple SSD.

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Is there any data available on how Remote Wipe works, in practice, and whether it's actually more effective at securely erasing an SSD than the techniques available via Disk Utility? –  Dan J Oct 3 '12 at 21:40
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@DanJ yes, you can read more about how remote wipe works in this thread. Apple specifically states that remote wipe works on FileVault protected SSDs by erasing the encryption key and other articles mentioned in that thread give other details. For a magnetic drive Disk Utility is almost certainly just as good (and easier to use) than remote wipe, but for SSDs it's not as clear. –  Old Pro Oct 5 '12 at 1:06
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