Reclaim lost disk space after deleting partition I tried to install Windows 10 to my iMac 10.13.4, through Boot Camp Assistant, but I failed to complete the process...

The BOOTCAMP partition was created though and I wanted to delete it. I realised I couldn't do it from the Disk Utility app, so I opened up the terminal and ran diskutil  eraseVolume  free  n  disk0s5 and the partition was deleted successfully!

After that, I realised that the space the BOOTCAMP partition occupied, wasn't released back to Macintosh HD! I searched around and found this command diskutil secureErase freespace 3 "/Volumes/Macintosh HD". I ran it, and by the time I'm writing this it needs another 15 hours.

Should I stop it? Will it even do what I want?
Thanks in advance!

PS: I know it's not ubuntu related, but since Macs are unix based, the answer should be the same
From this article it looks like what you're doing is a 35-pass secure wipe of the unpartitioned space.

You might have better luck resizing the existing Macintosh HD to the full disk capacity. 

If you try to resize Macintosh HD using disk utility and you encounter an error please update your question with a screenshot of the error, and the output of running diskutil list from a terminal window. Terminal can be found in the utilities folder.