Less free space after securely erasing free space? 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?
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 ?