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I use a mid-13 13-inches Retina MBP 256gb with macOS Sierra 10.12.2.

I recently decided to securely erase free space using the following terminal command:

diskutil secureErase freespace 1 /Volumes/HD

After completing its task, I got 50GB of free space instead of the 80GB I had before the operation.

Could it be a remaining temporary file used by secureErase?

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That's it. Do you have external SSD and FileVault enabled? Having SSD drive, Secure Erase and Erasing Free Space commands aren't needed for SSD drives 'cause a regular erase command makes it difficult to recover data from an SSD.

Also, if you don’t include freespace word, diskutil will start securely erasing the entire disk, instead of just the free space! 1 means single-pass method run of random numbers.

diskutil secureErase freespace 1 /Volumes/SSD

Be careful with it. Securely erased means there’s no chance you’ll be able to recover the data.

Read this useful AD post: Why is a secure erase 'not necessary' for SSD ?

  • The issue is having less space after wiping free space. I think it has something to do with some temp dummy files that haven't been removed properly after secureErase. – reaffer Jul 5 '17 at 2:50
  • This is considered unsafe. From the man " NOTE: This kind of secure erase is no longer considered safe." – gagarine Jan 28 at 18:18

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