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Occasionally I give away (or sell) an old Mac that I'm not using anymore. Before I do that, I want to make sure that none of my personal data remains on the drive, but I'm too lazy to reformat the drive and reinstall the OS. Instead, I do the following:

  1. Create a new Administrator account
  2. Log in using the new Administrator account
  3. Delete all the other existing user accounts (this step deletes all home folders, and therefore hopefully all personal information in those home folders)
  4. Open a Terminal window and enter this command:

    dd if=/dev/random of=bigfile.bin

  5. Let that command run until bigfile.bin has eaten up all of the free space on the drive, and the command terminates with an "error, out of disk space" message.

  6. Delete bigfile.bin to free up disk space again.

I think that for all practical purposes, filling up the hard drive's free space with random data should make it impossible for anyone to use an "undelete" utility to recover any files that were deleted in step 3.

The question is, is this process actually sufficient? Or is there something that I'm overlooking that could allow my personal data to "leak" despite the above? (Note that I don't consider the applications installed in the /Applications folder to be personal data)

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  • It is sufficient but you obviously know how you SHOULD do it. So the question is, if there is even the most remote chance of the recovery of your files, is it worth the risk and the opportunity cost of not doing it the way you KNOW you should do it worth it? We can't answer that question.
    – Ramhound
    May 22, 2015 at 16:29
  • Hey Jeremy, what you've done is sufficient for most people. Data recovery is still technically possible with a sector recovery program. But, it would take some time to get that deep. If you wanna be 100% sure, running a WIPE program across the drive 7 times will completely scatter the bits into oblivion. It's a bit over-kill but guaranteed data erased. You'd also have to re-install the MAC image when the data wipe was complete.
    – Tim
    May 22, 2015 at 16:30
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    just a note on licensing - if you give away a Mac, you can't also give away the applications; they're licensed to you. App Store apps especially are tied to your iCloud account & would need your credentials to update.
    – Tetsujin
    May 22, 2015 at 17:35

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