I figure if a government wants to hack my personal info, there are a zillion other avenues they would be more successful in hacking than a 3-pass-erasure of a secondhand SSD or hard drive that had encrypted personal data. In diskutil you have still Security Options in the Erase dialog.
There is a slider from Fastest to Most Secure. Whatever that means. It erases your files and writes over the data 7 times.
That should be enough. True; in this case you can't erase just the remaining free space, though—that only allows doing the writes over the entire disk. But I didn't realize the options were still there for full erase, thanks!
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When the erase has finished, click the Done button. But before you make use of the Security Options, make sure the volume being erased is part of a hard drive and not a fusion drive or an SSD. The Erase Security Options feature lets you pick the number of write passes that are made on the selected volume. If the volume you wish to erase is part of a hard drive, follow the steps outlined in Erasing a Volume: The Basics, above, until the erase sheet is shown.
At this point, click the Security Options button. The Security Options sheet will be displayed, showing a slider you can use to select one of four methods to erase the selected drive. The Fastest method is the same as the standard erase, and performs no special function to ensure any level of security. Picking this option will result in the volume being erased, but data can be recovered with basic data recovery apps. Moving the slider one click towards the right will produce the first secure erase, which writes a pass of random data followed by a pass of all zeros across the selected volume.
This two-pass write method will keep most individuals with prying eyes using conventional data recovery techniques from being able to access your old information.
Securely erase a Mac hard drive
The next security option produces a DOE-compliant three-pass secure erase. It uses two passes of random data followed by a third pass using a predefined data pattern. Using this method should secure your old data against most individuals, businesses, and governments not willing to spend excess money or time to uncover your data. The last and most secure option is a seven-pass erasure that meets DOD M standards for a secure wipe of magnetic media. As you advance the slider to more secure options, you also substantially increase the time it takes to perform the erase.
Make sure you really need this level of security before proceeding. By their physical nature, SSDs are inherently secure after a basic erase process.
Secure Erase OS X El Capitan Files & Disk Drives with Mac Wipe
The reason for this has to do with the internal architecture, logical to physical cell mapping, and wear leveling. Once the logical to physical map is removed, which occurs during a standard erase, accessing the internal data would just produce a hodge-podge of data that would be nearly impossible to decipher.
Erasing a volume by changing the format to include encryption, followed up by a second standard erase, will create a nicely sanitized volume with little chance of data recovery. A better and much faster method to produce a sanitized SSD volume is to encrypt the data on the drive, and then erase it, removing the stored encryption key along with everything else.
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This space sharing has a number of advantages, letting your volumes grow and shrink automatically as needed. But it could also have a security disadvantage. The shared space area of a container could hold old file data, even though the volume that originally contained the information was deleted.
In the case of an SSD, the logical to physical map detailing the location of any old file was removed when the volume was deleted. But three passes is what the US Department of Energy uses to securely delete their files. The Pentagon is even more paranoid: they use seven passes. Your Mac will begin wiping the drive. Both methods may adversely affect future performance of the drive, but are very effective at deterring file recovery efforts.
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