Choose the type of security you require ( see here for more information about erase methods).Select the volume whose free space you want to erase.Click Tools, and then click Erase Free Space.

To wipe your drive using Erase Freespace: Note: Wiping free space can take several hours, or even longer depending on the size of the drive, the number of passes selected, and the speed of the computer. Note: This process should not be used on any solid-state drive or other Flash-based storage. For privacy and security reasons, you can set CCleaner for Mac to wipe the free areas of your hard disk so that deleted files can never be recovered. This means that, given the right software, someone could reconstruct all, or parts of files that you've deleted. Over time, this data will be overwritten as macOS writes new files to that area of the drive. When you delete a file, macOS removes the reference to that file, but doesn't delete the actual data that made up the file on your hard drive. You can also find helpful information about the latest version of our macOS app. Note: this article is relevant to version 1.18.30 of CCleaner for Mac and below.

Free disk cleaner for mac