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So, recent versions of macOS no longer offer the option to Empty Trash Securely. From what I am reading here and here it seems the general consensus is that it is now best to encrypt files and then delete them.

However, if I create an encrypted disk image and move the files in there, it is using different blocks on the drive... so it does not zero out the blocks on the drive where the data was stored, and we're back where we started.

I've read comments from people saying deleted data is difficult to recover from an SSD anyway, so don't worry about it. Well even if that is true, what if you have a Fusion Drive and you don't even know which part of the drive the data in question resides in?

I'm not finding a clear answer on what the best way is to securely delete at present, which is surprising since it's a pretty common measure when transferring ownership of a computer, etc. Can someone please enlighten me:

  1. Is encrypting then deleting effectively the same as securely erasing?
  2. How can existing data be encrypted (rather than copied into an encrypted .dmg)?
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To answer your question 1: You often see encryption mentioned when you speak of securely deleting the contents of a computer.

The reasoning behind this is that if you for example fully encrypt the disk drive on your computer (i.e. use the built-in File Vault encryption as an example) - then no one can access the contents of the drive without the password.

If you then encrypt the drive with a long, random password and immediately forget that password - then you have effectively deleted the data. If you don't know the password, and it is not written down anywhere - noone can access the data.

The caveats are ofcourse if your password is so bad that it can be guessed, or there's a bug in the encryption routines that makes it possible to recover data without the password.

Remember that:

(a) You must not store or remember the password anywhere

(b) You must not add a password hint or anything that makes it possible to guess the password

(c) You should throw away any recovery keys or similar, and make sure you're not storing a copy of passwords/recovery key on iCloud

(d) The password should be sufficiently strong (i.e. long and random)

To answer your question 2: If you have already stored the data unencrypted, you're out of luck. Just copying them into an encrypted image or anything like that won't replace the data on disk. Instead you'll be making an encrypted copy of your data - there's no guarantee that the disk will really delete the non-encrypted data.

  • Thank you for the detailed answer. Regarding: "If you have already stored the data unencrypted, you're out of luck." Do you know why this was not so true in earlier versions of macOS that included the option to empty trash securely? Surely zeroing out the data is better than nothing, even if it's not as secure as storing it encrypted to begin with? I'm surprised the feature was dropped completely. – Mentalist Mar 6 '18 at 15:05
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    If you store a file on a hard disk drive, and you rewrite the contents of that file - you'll be writing over the same physical part of the disk drive. If you do that multiple times, any physical signs of the old data is gone. It is simply not possible with todays technology to recover that date. Secure delete works there. With an SSD things are different. Writing over a file most likely do not write over the same physical parts of the SSD because of its wear levelling algorithms. That means that you'll leave behind a copy of the unencrypted data. – jksoegaard Mar 6 '18 at 15:14
  • Thanks. Okay, that makes sense. So what about people using older Macs that have SSDs with versions of OS X that still had the option to empty trash securely? Was the secure erase just not very effective on those drives, or what? – Mentalist Mar 6 '18 at 15:27
  • Yeah, you could say that. Not very effective = effectively not working at all. However, if you had a Mac with an ordinary hard disk drive, secure erase is still a valid approach. – jksoegaard Mar 6 '18 at 15:32
  • So for someone who has a fairly current Mac that needs to get data from a much older Mac running Snow Leopard (unable to read encrypted volumes) the best thing to do would be to copy the data once to an external HDD, then copy that data to an encrypted volume (another partition possibly), then reconnect the HDD to the Snow Leopard Mac, trash the data, and securely empty that drive's Trash using Snow Leopard. Would you agree? – Mentalist Mar 6 '18 at 15:42

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